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Old 17th August 2006, 05:19 AM   #11
Jennice is offline Jennice  Denmark
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I found out that the main issue with Creative cards is that they resample everything from 44.1 to 48 kHz, which causes problems for accurate measurements and phase testing.

M-audio also makes the Audiophile card, which has external PSU (not feeding from the USB power). The specs look impressive, but the cheap Transient card seems to have decent SNR specs, too.

I checked with LspCAD, and it supports 48 kHz sampling rate, which seems to be the usual for M-audio's cards.
However, it also supports just about any other imaginable rate, so I guess this leaves a free choice, if Creative's main problem is their re-sampling from 44.1 to 48 kHz.

What's this latency issue with USB devices? What does it mean? (I guess my english isn't good enough to understand). Can anyone explain?

Jennice
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Old 17th August 2006, 04:56 PM   #12
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Default higher latency of USB?

" ... at's this latency issue with USB devices? What does it mean? (I guess my english isn't good enough to understand). Can anyone explain? ..."

All electronic data transfer media, analog or digital, suffer from "latency" because of the speed of light and sound (signals) in metal conductors.

Basically, because of the nature of USB being "full duplex" (two data wires) and FireWire being "quad duplex" (four data wires), there are latency issues with USB. These have been well known for about a decade. This problem applies to all "packet switching networks" in varying degrees (USB, FireWire, modern EtherNet, SATA, etc. ... and even though modern EtherNet has eight data wires, the packet switching and handshaking overhead of EtherNet increases latency to unmanagable levels for DAC, ADC audio I/O).

Professional studio operators originally had tough sledding trying to feed USB to the control room for digital recording, then feeding digital to analog audio (DAC) back to the studio for double tracking, etc. ... especially when adding voice tracks. The musician (as listener) hears a tiny delay (the latency) and when trying to sing along (adding tracks) this latency results in "out of sync" tracks (late arriving = latency). There is no perfect solution, yet ... and the use of "simpty time codes" to square it up does not help the musician a bit.

With FireWire the latency is greatly reduced, enough so that the latency is almost undetectable by the musicians and studio operators. This is why so much of the professional studio digital to analog (DAC) and analog to digital converters are connected via FireWire (IEEE 1394a or iLink (Sony)) rather than by USB or some other scenario. It is still quite common in the professional studio to do everything as analog, first, then make the digital conversion at the final mix down .... unless the studio is completely "FireWired", then all original sources are converted to digital from the microphones and pickups directly to the digital tracks.

Important: The larger latency of USB for computer to analog audio output is not a problem for those of us looking for quality audio playback ... only. It is only of concern with bi-directional audio interfaces, analog to digital and digital to analog, (audio in & audio out), and even then there are ways to keep it all straight, unless you are doing live recording and then "double tracking".

References:
http://en.wikipedia.org/wiki/SMPTE_time_code

Compare the specs of these for clarity:
http://rolandus.com/products/product...1&ParentId=114 ... verses ... http://rolandus.com/products/product...4&ParentId=114 ...
http://www.m-audio.com/products/en_u...kPro-main.html ... verses ... http://www.m-audio.com/products/en_u...hile-main.html ...

http://www.oxsemi.com/oxford/documen...dio/audio.html

FYI: FireWire 400 is more than twice as fast as USB 2.0 and greater bandwidth than 1000baseT "gigabit" Ethernet by about 10% ... in "bulk file transfer modes", ala music and video. FireWire 800 (1394b) is the fastest and has the smallest latency, being faster than any scenario and primarily used for video graphics rendering / networking = about 2.5 times as fast as Gigabit Ethernet. (Check out movies "Sin City" or "Sky Captain" = all FireWire 800 graphics rendering on Apple Macs. Then check out Maya software & hardware rendering scenarios.)

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Old 17th August 2006, 05:49 PM   #13
Jennice is offline Jennice  Denmark
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Hi FastEddy,

Thanks for your good explanation of latency.

In my case I don't think latency is an issue, as am not comparing the response (input to the sound card) to the output from the sound card.
I will be measuring the drive signal with a probe feeding the left input, and the microphone response (through a seperate preamp) on the right input channel.

I have yet to investigate if it becomes a problem during calibration, as calibration is a "loop" from output back to input. A significant delay in this chain will result in a phase issue.
Then again, as long as this calibration is aware of a phase shift, and the same applies to the real measurement, it should be ok... I think!
I have yet to think more about this. Having to add firewire is a costly addition.
although less flexible, I have also seen manufacturers of sound cards for the PC-bus (32 bit version of the PCMCIA). I suppose bandwidth is good enough there to have little latency concerns - or am I mistaken?

Jennice
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Old 17th August 2006, 07:07 PM   #14
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Default get it right the first time?

We always recommend the FireWire interface as a matter of course, whether output only or bi-directional input / output.

But, USB generally is a little less expensive.

As for " ... I have also seen manufacturers of sound cards for the PC-bus (32 bit version of the PCMCIA) ...", the latency problems may disappear, but the power supply noise problem then becomes an issue ... as per previous postings.

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Old 17th August 2006, 07:18 PM   #15
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Default blatent commercial announcement digital I/O

See http://industrialcomponent.com/firew.../fws46603.html ... we have been giving this plugin card "kit" away for free with the purchase of any M-Audio FireWire device ( http://industrialcomponent.com/maudi...recording.html )

Also that M-Audio Transit is a fine way to isolate PC power from audio devices via Toslink optical ... USB output only = no latency concerns. ( http://industrialcomponent.com/maudio/us99130.html )

....
Disclosure: I work for these folks: http://industrialcomponent.com/index.html ... so ...
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Old 18th August 2006, 04:51 AM   #16
Jennice is offline Jennice  Denmark
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FastEddy,

Even though the add about giving away the PCI cards with M-audio products was a little misleading (it's for a laptop), I appreciate your feedback on this issue.

I've found a (fairly) cheap PC-card bus adaptor with USB2 and 2 x FireWire400 ports. Would that be a solution if connected to a FireWire sound card?

Jennice
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Old 18th August 2006, 05:01 AM   #17
Jennice is offline Jennice  Denmark
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FastEddy,

I looked at some FireWire sound cards
That's expensive stuff (couldn't find M-audio products locally with FW)!
The only thing I can afford (of what I've seen) is the Behringer FCA-202.

I think I'm at the point where I have to consider if it's worth it, and dig more into the latency issue (if it's any problem for me).

Thanks for your ideas and feedback so far!

Jennice
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Old 18th August 2006, 03:35 PM   #18
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Default actually .... USB does a great job ...

... but, if you are interested in Professional, Studio Quality results, especially when it comes to dubbing, mixing tracks, adding live tracks, sophisticated multi track work, etc. ... then consider FireWire ...

Do Not Discount the abilities of the better USB sound interfaces verses built in or plug in sound cards.

Generally, digital playback is as good via the USB methodology as any other technology and usually much better than "built in" or plug in sound cards, PCI, PCMCIA (CardBus) or what have you ... Mostly because of the nature of the mass market PC and Laptop power supplies and built in or plug in PC power filtration. (PCs and Laptops all use switching power supplies, which are notoriously dirty animals = lots of noise, distortion, wheat chaff and oat husks ... )

If you want SPDIF coaxial, digital to digital output, then the external USB devices are superior to the built in and plug in cards, period. (Digital to digital output, the US$100 M-Audio Transit USB device is vastly superior to any built in or plug in sound card.)

If you want SPDIF coaxial and SPDIF optical output (Toslink, et al) then the built in and plug in cards may or may not suffice, but for reasons of cross compatibility, usefulness, versitality and resale value, again, the external USB devices are superior to any built in and plug in cards.

If you want complete, bi-directional (input & output) of digital to analog and analog to digital audio, then there is absolutely nothing wrong with the better quality external USB audio devices. (The latency question as originally addressed here is eleviated better with FireWire 1394, but this latency question does not affect good quality, standard playback and/or capture for ripping, burning or pure digital mix down ... just when mixing live analog input to previously recorded digital.)

Personally, I have a new Apple MacBook with the SPDIF optical port built in ... and it serves the purpose very well indeed, transporting 24 bit x 96k audio out to my optical port equipped playback system. (All Apple MiniMacs, iMacs, G5s, PowerBooks and the rest all are now so equipped = built in SPDIF optical ... plus FireWire ports ... plus several USB ports.)

If I were to take this Laptop into a studio for some serious mixing and/or live recording, I would take along an M-Audio FireWire devices ( http://www.m-audio.com/products/en_u...hile-main.html ) or Roland FireWire device ( http://rolandus.com/products/product...1&ParentId=114 ) ... primarily because that pesky latency question is fully addressed or at least resolved to the point on non-contention.

If I were to use this Laptop for simple, serious digital to analog (DAC) or serious SPDIF coaxial playback into a top quality solid state or tube amplified sound system, I would get the well filtered, well engineered WaveLength Audio devices USB to DAC ( http://www.wavelengthaudio.com/usbdac.html )

Thruth is one, paths are many ...
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Old 19th August 2006, 08:26 AM   #19
Jennice is offline Jennice  Denmark
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FastEddy,

My purpose is purely the testing of speakers. The only iussue I will have to consider is this latency, and if it makes any difference at all, if there is a delay between the cards analog output, and the sampling of the analog inputs.

I will have to dig into this issue a little more.

However, I want to thank you for all your replies to my (possibly stupid) questions about latency and similar.

I think it's time for me to sit down and think this through, and ask some questions to the software manufacurer.


Jennice
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Old 19th August 2006, 11:02 PM   #20
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Default Latency ....

Consider:

If your "speakers under test" are connected to your amp, your amp to a pre-amp, your pre-amp has a (or is connected to) a SPDIF or other type of Digiral to Analog converter, your pre-amp connected DAC is then connected to your computer .... AND the sound from those speakers is recorded by your microphones to an Analog to Digital converter (same as above or other channel) ... and that analog to digital converter (A/D) is .... feed into the same computer = playback back output and bi-directional conversion D to A and then A to D back to the same computer ....

Then latency is inevitble ... and no matter what transport and/or conversion type ( methodology = USB, PCMCIA direct, plug in sound card or built in optical or FireWire ) is used ... latency will show up.

In fact latency of a very modest amount would be present whether your test system is pure analog or pure digital or a combination of the two (as is now more common practice). This is the result of the speed of sound in the air between the speakers and the microphones and the speed of light (electronic audio signals) in metal or glass conductors ... By adding all the computer processors' process / conversion times of the(internal or external) chips to the total latency ... then this may become significant enough to cause errors in your tests.

The best measurement tests of audio equipment as you are trying to do are accomplished with "pure" analog equipment = pre-amp and amp, speakers and microphones ... analog osciloscopes, spectrum analisers and signal generators.

Second best for test of speakers are "pure" analog sources played back and captured to
via bi-directional A to D and D to A converters of highest possible resolution and the Least Latency possible ... AND introducing software timing corrections to the analisys ... like spmte time code corrections in programs like "Pro Tools", etc.

Of interest: many of the better osciloscope hardware and software scenarios do a very good job of signal generation and reporting and especially display of spectrum analisys and insertion of various schemes to account for Latencies.

Of course you can use consumer and commercial and audiophile grade add-ons to your computer system, but chosing the best ways to reduce or limit Latency problems have already been described abov. My opinion backed by professionals is that FireWire interfaces for conversion of both input and output ... has the least latency problems of all.

Perdon my spelling (sic)

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