Up until very recently I thought of near-perfect reproduction as being unreachable and unaffordable and (in a way) a bit unnecessary.
Recently I came across this
http://www.linkwitzlab.com/reference_earphones.htm
Which led to this
http://www.johncon.com/john/amp/
I have ordered a set of the Shure in-ear phones on the strength of this. The Sony phones are available at very reasonable prices so I’ll probably get a set of these too. Inevitably I will build the amp, as the cost is trivial.
If you read the rest of what these guys have exposed on the web there's good reason to think that these items (the phones and amp) in combination will provide a quality of reproduction against which others can be judged.
Of course in-ear phones are not ideal for all listening, but it would be nice to be able to tell exactly where the flaws in something played back are coming from. For example, is it the recording? You can play the same source through different chains to get this kind of insight, but it is timeconsuming.
Now all I need is an unimpeachable digital source...
I have read a lot of posts about digital sources. While there is considerable excitement about higher definition recording, the majority of recorded material currently available is 44k1 16-bit and indeed many authorities insist that the standard is fully adequate if exploited to the full. Increasing numbers of people seem to accept that computers make excellent sources for audio. Where there are concerns they seem to centre around jitter. I find this difficult to understand, since this seems to me easy to eliminate, but without investigating the circuits of the commercially available USB and other DACs it is difficult to know to what extent the concerns are justified.
In order to sidestep these issues, and in the interest of providing a well understood and documented process whereby the digital information is converted to analog, I am thinking of constructing a player.
This would be a simple device stripping uncompressed linear PCM from a flash memory device and feeding it to a DAC. A high-quality low-phase-noise TCXO would be the clock source and should eliminate any concerns about jitter. No operating system as such would be required, a simple vending-machine style finite state machine could be implemented in a small FPGA or CPLD.
It is a shame that sufficiently fast devices are not available since a full-resolution PWM DAC could otherwise also be accommodated and would thoroughly satisfy my taste for knowing exactly what is going on in a particular implementation.
Be that as it may, given the construction of the device, data could for example be read from a CD on a PC, cross checked against a second CD if this was felt to be necessary, and uploaded to a flash memory which could then be played back in as near to jitter-free a fashion as imaginable without resorting to extraordinary measures.
I was impressed by the low noise performance of Tektronix hand-held scopes when run on their internal batteries, so I would probably aim at running the whole shebang from batteries.
Anyway I figure I can lash this up fairly readily.
If a suitable DAC is selected then the system might have its usefulness extended to accommodate future formats including 24-bit and 192kHz although a second TCXO may be required. Given that the logic is reprogrammable this need not be addressed immediately and prior provision could be made on the PCB.
There is also a case for a NOS 16-bit device dependent on linearity and other factors given that the intrinsic SNR of most program material is 96dB anyway. Any comments about reconstruction filtering will also be appreciated.
Flash memory of 16G is available, so if a FAT reader and some user interface were provided the device would be useful for more than just test purposes, although any additional functionality adds to the problems of keeping the system-generated noise low.
None of these components are exorbitantly expensive.
I am prepared to hear arguments that, for example, an iPod of a particular vintage with lossless compression is adequate to my requirements, however I would like to hear any recommendations as to which (chip) DAC might be employed.
Or perhaps someone else knows if there's any way of getting what I want, or close to it, without spending an unconscionable amount of money?
w
Recently I came across this
http://www.linkwitzlab.com/reference_earphones.htm
Which led to this
http://www.johncon.com/john/amp/
I have ordered a set of the Shure in-ear phones on the strength of this. The Sony phones are available at very reasonable prices so I’ll probably get a set of these too. Inevitably I will build the amp, as the cost is trivial.
If you read the rest of what these guys have exposed on the web there's good reason to think that these items (the phones and amp) in combination will provide a quality of reproduction against which others can be judged.
Of course in-ear phones are not ideal for all listening, but it would be nice to be able to tell exactly where the flaws in something played back are coming from. For example, is it the recording? You can play the same source through different chains to get this kind of insight, but it is timeconsuming.
Now all I need is an unimpeachable digital source...
I have read a lot of posts about digital sources. While there is considerable excitement about higher definition recording, the majority of recorded material currently available is 44k1 16-bit and indeed many authorities insist that the standard is fully adequate if exploited to the full. Increasing numbers of people seem to accept that computers make excellent sources for audio. Where there are concerns they seem to centre around jitter. I find this difficult to understand, since this seems to me easy to eliminate, but without investigating the circuits of the commercially available USB and other DACs it is difficult to know to what extent the concerns are justified.
In order to sidestep these issues, and in the interest of providing a well understood and documented process whereby the digital information is converted to analog, I am thinking of constructing a player.
This would be a simple device stripping uncompressed linear PCM from a flash memory device and feeding it to a DAC. A high-quality low-phase-noise TCXO would be the clock source and should eliminate any concerns about jitter. No operating system as such would be required, a simple vending-machine style finite state machine could be implemented in a small FPGA or CPLD.
It is a shame that sufficiently fast devices are not available since a full-resolution PWM DAC could otherwise also be accommodated and would thoroughly satisfy my taste for knowing exactly what is going on in a particular implementation.
Be that as it may, given the construction of the device, data could for example be read from a CD on a PC, cross checked against a second CD if this was felt to be necessary, and uploaded to a flash memory which could then be played back in as near to jitter-free a fashion as imaginable without resorting to extraordinary measures.
I was impressed by the low noise performance of Tektronix hand-held scopes when run on their internal batteries, so I would probably aim at running the whole shebang from batteries.
Anyway I figure I can lash this up fairly readily.
If a suitable DAC is selected then the system might have its usefulness extended to accommodate future formats including 24-bit and 192kHz although a second TCXO may be required. Given that the logic is reprogrammable this need not be addressed immediately and prior provision could be made on the PCB.
There is also a case for a NOS 16-bit device dependent on linearity and other factors given that the intrinsic SNR of most program material is 96dB anyway. Any comments about reconstruction filtering will also be appreciated.
Flash memory of 16G is available, so if a FAT reader and some user interface were provided the device would be useful for more than just test purposes, although any additional functionality adds to the problems of keeping the system-generated noise low.
None of these components are exorbitantly expensive.
I am prepared to hear arguments that, for example, an iPod of a particular vintage with lossless compression is adequate to my requirements, however I would like to hear any recommendations as to which (chip) DAC might be employed.
Or perhaps someone else knows if there's any way of getting what I want, or close to it, without spending an unconscionable amount of money?
w
OK, this is a suggestion at least.
I am aware of the existence of the Squeezbox and its big brother the Transporter (prices starting at $2k US).
What I'm aiming at is more a cheap tool than an entertainment system.
There's too much room to argue about packets lost over the network with this kind of streaming device, and the operating system is too complex, even if full details were available, for the whole process to be readily transparent even to someone with considerable expertise. I have little doubt that the Slim Devices products are well enough engineered not to have any problems with buffer underruns or jitter, but my confidence is not equivalent to being able to say that the device design is well documented, or even highly regarded by purchasers.
I'm talking about building something so simple its operation could be understood almost as readily as a turntable by observation.
Buyers are confronted with a difficult decision when considering purchases. Devices generally come with a technical specification, and are tested for compliance before release. (Release test for some items is becoming increasingly perfunctory.) Other than this there are reviews and a great number of anecdotes published on the web.
Given that numerous producers are competing for business and that marketing departments are occasionally unfussy in the strategies they employ, it is unfortunately necessary to beware that not everything that one reads is written in good faith.
Is the Transporter really audibly better than the Squeezebox? Who can tell without buying both? ...not really an option as far as I'm concerned.
I am fairly sure that a number of the available DACs have performance errors below the level of perceptibility as determined by tests on a human population. This is, however, not my area of expertise, although I am prepared to make it so.
If a well documented system were available, then listening and other tests of any system against the reference system could eliminate concerns about its adequacy.
There are very many DACs on the TI site alone. Perhaps somebody knows which ones have a poor reputation?
w
I am aware of the existence of the Squeezbox and its big brother the Transporter (prices starting at $2k US).
What I'm aiming at is more a cheap tool than an entertainment system.
There's too much room to argue about packets lost over the network with this kind of streaming device, and the operating system is too complex, even if full details were available, for the whole process to be readily transparent even to someone with considerable expertise. I have little doubt that the Slim Devices products are well enough engineered not to have any problems with buffer underruns or jitter, but my confidence is not equivalent to being able to say that the device design is well documented, or even highly regarded by purchasers.
I'm talking about building something so simple its operation could be understood almost as readily as a turntable by observation.
Buyers are confronted with a difficult decision when considering purchases. Devices generally come with a technical specification, and are tested for compliance before release. (Release test for some items is becoming increasingly perfunctory.) Other than this there are reviews and a great number of anecdotes published on the web.
Given that numerous producers are competing for business and that marketing departments are occasionally unfussy in the strategies they employ, it is unfortunately necessary to beware that not everything that one reads is written in good faith.
Is the Transporter really audibly better than the Squeezebox? Who can tell without buying both? ...not really an option as far as I'm concerned.
I am fairly sure that a number of the available DACs have performance errors below the level of perceptibility as determined by tests on a human population. This is, however, not my area of expertise, although I am prepared to make it so.
If a well documented system were available, then listening and other tests of any system against the reference system could eliminate concerns about its adequacy.
There are very many DACs on the TI site alone. Perhaps somebody knows which ones have a poor reputation?
w
I know this is only picking on a tiny subsection of your enquiry, but:
What do you mean, there? Pretty much any modern FPGA is fast enough to make a 24-bit resolution PWM DAC. I imagine one of the cheap Digilent dev boards would be suitable for this, and could just be included in your main enclosure - or, if you have the know-how (I don't), you could roll your own custom FPGA board. Either way, excellent results could definitely be attained.
What do you mean, there? Pretty much any modern FPGA is fast enough to make a 24-bit resolution PWM DAC. I imagine one of the cheap Digilent dev boards would be suitable for this, and could just be included in your main enclosure - or, if you have the know-how (I don't), you could roll your own custom FPGA board. Either way, excellent results could definitely be attained.
The idea to read the audio samples from a RAM, which is clocked by the same low jitter clock as the DAC, is indeed a way to essentially eliminate the jitter problem. And large flash memories are now very affordable,
Teac is using a 128 MB RAM-Buffer in its D-70 DAC unit, see www.teac.com/esoteric/D-70.html. I also read about at least one other DAC which reads the entire CD into RAM before playing it, but have unfortunately lost that link.
As far as DACs are concerned, the new ESS Sabre chips are state of the art; there is a thread right here in the digital section. Chips from Wolfson are also highly regarded.
Kurt
Teac is using a 128 MB RAM-Buffer in its D-70 DAC unit, see www.teac.com/esoteric/D-70.html. I also read about at least one other DAC which reads the entire CD into RAM before playing it, but have unfortunately lost that link.
As far as DACs are concerned, the new ESS Sabre chips are state of the art; there is a thread right here in the digital section. Chips from Wolfson are also highly regarded.
Kurt
wakibaki said:
Just make sure you don't reinvent the wheel:
http://www.diyaudio.com/forums/showthread.php?s=&threadid=85473
you mean this?
http://www.diyaudio.com/forums/showthread.php?s=&threadid=90725&highlight=
..... with a TCXO/VCXO ?
http://www.diyaudio.com/forums/showthread.php?s=&threadid=90725&highlight=
..... with a TCXO/VCXO ?
No no, Sigma-Delta is a type of Pulse Density Modulation - altogether different. Sigma-Delta is really quite ugly, mathematically problematic, and almost always unstable. I'm not a fan.
At its most basic, PWM can be implemented as Uniformly-sampled PWM (UPWM) with a counter:
- Pre-load the counter with the current sample's value,
- Set the PWM output HIGH at the beginning of the current sample,
- Wait for a bit,
- Set the output LOW once the counter reaches zero,
- Wait until the end of the current sample, and repeat.
This doesn't work for full-resolution audio because the counter clock rate has to operate at 2^N times the basic sample-rate, for N-bit audio. 2^16 * 44100 is 2.89Ghz - not feasible. Additionally, UPWM generates truly silly levels of harmonic and intermodulation distortion, the amount of which gets worse as the input frequency approaches the switching rate. Nobody uses PWM like this.
The simplest and most obvious approach is to oversample and noise-shape the input data. Oversampling raises the switching rate, and so keeps the input signal much lower than it - it thus helps a bit with distortion; noise-shaping allows a large dynamic range within the small bandwidth of interest by pushing the quantisation noise up into the new regions created by oversampling. This sort of scheme makes PWM at least a workable paradigm. A 100MHz internal clock rate is entirely suitable and around 16-bit performance can be achieved at 44.1kHz with this.
For higher levels of performance, you need to directly attack PWM's distortion via some sort of precompensation system, and lots of methods have been developed in the AES (that are far too complicated to describe here) - with excellent results. At the top-end, one scheme achieves a theoretical 26 bits of distortion-free modulation performance in the passband, with either a 4-bit or a 6-bit (can't quite remember!) UPWM modulator. Impressive stuff!
If you have access to the journals I can give you some interesting references to check out. They're all (or, nearly all) protected by patents - but I don't imagine that's a problem for personal use.
). I'll have to review my discrete-time signals and systems material, urgh. Thanks for the pointers, I really appreciate it.AES journals on PWM (applied to digital power amplifiers). This is really just a list of highlights - it's often worth checking out their references list too.
Sandler, et al; 1991:
"Developments in Realising an All-Digital Power Amplifier"
Standard noise-shaped PWM - a good introduction to PWM and its limitations
Sandler, et al; 1991:
"Ultra-Low Distortion Digital Power Amplification"
Pseudo-Naturally Sampled PWM - a PWM linearisation scheme that works by converting UPWM into something that approximates the (distortionless) way analogue Class-D amplifiers do it
Hawksford; 1992:
"Dynamic Model-Based Linearisation of Quantised Pulse-Width Modulation for Applications in Digital-to-Analogue Conversion and Digital Power Amplifier Systems"
Another PWM linearisation scheme based on dynamic FIR filtering to compensate for the frequency response abberations caused by PWM pulses
Craven; 1993:
"Toward the 24-bit DAC, Novel Noise-Shaping Topologies Incorporating Correction for the Non-Linearities in a PWM Output Stage"
This appears to be the most effective PWM linearisation scheme yet proposed. Beautiful, but pretty crazy complicated. Have not yet spent the time in getting my head around it properly.
This lot should be a good introduction to PWM. It's so much nicer than Sigma-Delta, I don't know why it's not in more widespread use. I plan on rolling my own FPGA-based PWM DACs sometime soon (once I finish uni and have some time!)
Sandler, et al; 1991:
"Developments in Realising an All-Digital Power Amplifier"
Standard noise-shaped PWM - a good introduction to PWM and its limitations
Sandler, et al; 1991:
"Ultra-Low Distortion Digital Power Amplification"
Pseudo-Naturally Sampled PWM - a PWM linearisation scheme that works by converting UPWM into something that approximates the (distortionless) way analogue Class-D amplifiers do it
Hawksford; 1992:
"Dynamic Model-Based Linearisation of Quantised Pulse-Width Modulation for Applications in Digital-to-Analogue Conversion and Digital Power Amplifier Systems"
Another PWM linearisation scheme based on dynamic FIR filtering to compensate for the frequency response abberations caused by PWM pulses
Craven; 1993:
"Toward the 24-bit DAC, Novel Noise-Shaping Topologies Incorporating Correction for the Non-Linearities in a PWM Output Stage"
This appears to be the most effective PWM linearisation scheme yet proposed. Beautiful, but pretty crazy complicated. Have not yet spent the time in getting my head around it properly.
This lot should be a good introduction to PWM. It's so much nicer than Sigma-Delta, I don't know why it's not in more widespread use. I plan on rolling my own FPGA-based PWM DACs sometime soon (once I finish uni and have some time!)
Having played with 8-bit PWM audio I think it's over-hasty to dismiss this approach. Anyway I was just really trailing my coat.
I have listened to a number of sources with these excellent (although, I discover, not top-of-the-range) in-ear Shure phones, I am doubtful of the value to me of the player I originally thought about building. The quality of reproduction from the several DACs I have in various domestic devices is so good that although they are not identical, it is hard to identify differences or serious deficiencies, let alone pick a favourite.
When it comes down to it, despite all the audiophile hooraw, when you sit down to design one of these things there's nothing much to fall back on other than the datasheets and the measurements. Boring.
A few years ago playback on a computer was likely to be interrupted by some process with a higher priority. Now I can make damn good glitch-free multi-track recordings. The main problem is the noise from the fan.
I have just been looking at the E-MU 1616M PCMCIA and PCI audio interfaces which have been out for over a year,although I hadn't seen them 'til now. These have 6 x 24/192 outs. I could connect these up to 3 T-amps and independently drive a 3-way speaker system with no crossover losses and fiddle with the cutoffs and overall balance using a recording package to import and play back CDs.
Plus - it records, has phantom power, mic preamps, guitar input and a slew of other desirable features.
I'm a long time out of uni, and I've built a lot of things but this time I guess I’ll get one of these to play with instead and try to keep the worst of my obsessive characteristics under control. Anybody got one?
I have listened to a number of sources with these excellent (although, I discover, not top-of-the-range) in-ear Shure phones, I am doubtful of the value to me of the player I originally thought about building. The quality of reproduction from the several DACs I have in various domestic devices is so good that although they are not identical, it is hard to identify differences or serious deficiencies, let alone pick a favourite.
When it comes down to it, despite all the audiophile hooraw, when you sit down to design one of these things there's nothing much to fall back on other than the datasheets and the measurements. Boring.
A few years ago playback on a computer was likely to be interrupted by some process with a higher priority. Now I can make damn good glitch-free multi-track recordings. The main problem is the noise from the fan.
I have just been looking at the E-MU 1616M PCMCIA and PCI audio interfaces which have been out for over a year,although I hadn't seen them 'til now. These have 6 x 24/192 outs. I could connect these up to 3 T-amps and independently drive a 3-way speaker system with no crossover losses and fiddle with the cutoffs and overall balance using a recording package to import and play back CDs.
Plus - it records, has phantom power, mic preamps, guitar input and a slew of other desirable features.
I'm a long time out of uni, and I've built a lot of things but this time I guess I’ll get one of these to play with instead and try to keep the worst of my obsessive characteristics under control. Anybody got one?
That's basically what I'm doing. Different amps and soundcard, but the same basic idea. I use a dedicated PC for the crossovers, which I find is a lot nicer than having everything in one machine because you can leave it on and forget about it, and there's no chance of random OS sounds interrupting things and/or frying your tweeters.
In fairness, while making custom stuff sounds interesting and fun, off-the-shelf stuff like a PC is certainly much easier for experimenting with. And you can't beat the flexibility of such a setup. It definitely gets a
from me. Fan noise isn't even a real problem with a little extra effort on a quiet cooling solution.This thread is most interesting. I have a couple of things to contribute.
I have a set of Shure E3 in ear monitors. they are so good I had to dump my entire 256k-bit MP3 library and re-record as wav. I find the foam pads very uncomfortable and have had to use the smallest rubber plugs. My ear canals are unusually small in diameter.
It may or may not be related but my high frequency hearing is very good, pretty much flat to 19.5 kHz. At the age of 16 it was good to 22.5kHz.
I have experiemented with a few T-amps. I find the treble unnatural and have abandoned the concept. Many people love them and they might be ok for you.
Bert Doppenberg of BD design has made what he considers the ultimate DAC. It has flash memory, the file is uploaded to the DAC from a PC, once the upload is complete the music is played from the flash memory. Have a look on his website for details. I can't afford it but I'm sure it's worth the price.
I bought a USB Monica recently. I like it. treble is natural. It's cheap, it only does up to 16/48kHz. there are some unusual elements to the design.
I use a mac mini for the source, the fan noise is inaudible.
I'm looking for a portable source of high quality and large storage, something like an Ipod but the new DACs dont appear to be very good. I'll be watching this thread to see what you finish up doing.
I have a set of Shure E3 in ear monitors. they are so good I had to dump my entire 256k-bit MP3 library and re-record as wav. I find the foam pads very uncomfortable and have had to use the smallest rubber plugs. My ear canals are unusually small in diameter.
It may or may not be related but my high frequency hearing is very good, pretty much flat to 19.5 kHz. At the age of 16 it was good to 22.5kHz.
I have experiemented with a few T-amps. I find the treble unnatural and have abandoned the concept. Many people love them and they might be ok for you.
Bert Doppenberg of BD design has made what he considers the ultimate DAC. It has flash memory, the file is uploaded to the DAC from a PC, once the upload is complete the music is played from the flash memory. Have a look on his website for details. I can't afford it but I'm sure it's worth the price.
I bought a USB Monica recently. I like it. treble is natural. It's cheap, it only does up to 16/48kHz. there are some unusual elements to the design.
I use a mac mini for the source, the fan noise is inaudible.
I'm looking for a portable source of high quality and large storage, something like an Ipod but the new DACs dont appear to be very good. I'll be watching this thread to see what you finish up doing.
Hi OzMike
'the file is uploaded to the DAC from a PC, once the upload is complete the music is played from the flash memory'
This is basically what I had in mind. Although there is a considerable amount of work involved, there is no reason the components should be exorbitantly expensive.
I lost interest in this as a project tho'...
1. Nobody else seems to give a damm.
2. I don't believe there will be an audible difference between this and a well-constructed conventional PC interface. It was just an easy to eliminate the possibility of significant jitter for those who are convinced of the need.
3. The only independent objective DAC reviews I can find date from the late 90's.
In the latter part of the 35mm camera era, some review magazines stopped publishing lens performance data. This was partly due to the increasing rarity of poorly performing lenses in a mature market. This has come back with digital cameras, but this technology is immature.
I suspect that the performance of most recent chip DACs well exceeds the threshold of audibility for all listeners.
Must be nice to know you can hear that high. I go up to 15k5 both ears. OK for 57.
w
On the subject of PWM, a thought experiment. Think about the output switches in a T-amp. These turn on and off on a timeline. You could play a piece of music through the amplifier and, if you could record the transitions with sufficient resolution, you could then replay the sequence of transitions through the same switches...
Now imagine you can simulate the T-amp in software.
'the file is uploaded to the DAC from a PC, once the upload is complete the music is played from the flash memory'
This is basically what I had in mind. Although there is a considerable amount of work involved, there is no reason the components should be exorbitantly expensive.
I lost interest in this as a project tho'...
1. Nobody else seems to give a damm.
2. I don't believe there will be an audible difference between this and a well-constructed conventional PC interface. It was just an easy to eliminate the possibility of significant jitter for those who are convinced of the need.
3. The only independent objective DAC reviews I can find date from the late 90's.
In the latter part of the 35mm camera era, some review magazines stopped publishing lens performance data. This was partly due to the increasing rarity of poorly performing lenses in a mature market. This has come back with digital cameras, but this technology is immature.
I suspect that the performance of most recent chip DACs well exceeds the threshold of audibility for all listeners.
Must be nice to know you can hear that high. I go up to 15k5 both ears. OK for 57.
w
On the subject of PWM, a thought experiment. Think about the output switches in a T-amp. These turn on and off on a timeline. You could play a piece of music through the amplifier and, if you could record the transitions with sufficient resolution, you could then replay the sequence of transitions through the same switches...
Now imagine you can simulate the T-amp in software.
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