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DIY 7.1 over Spdif?
DIY 7.1 over Spdif?
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Old 28th September 2010, 02:58 PM   #11
macboy is offline macboy  Canada
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Quote:
Originally Posted by Xm0dgeek View Post
I don't mind if the signal is compressed as long as it's not markedly bad sounding. Can some one point me towards some more info on the subject? I know it has to be possible or else how can you get 7.1 over normal consumer SPDIF? I'm just looking for it diy.
HDMI LPCM 7.1 5.1 Multi-Channel Audio Decoder Amplifier - eBay (item 350247932081 end time Oct-27-10 13:28:24 PDT)
This is basely what I want to make right here. I don't really car about the Dolby or DTS stuff (tho it would be nice) I'm just wanting to hook up my PC to it.

It says it supports 8 channel LPCM but only with HDMI so I'm guessing HDMI has multiple digital audio lines then?
You can't do 7.1 over S/PDIF. Neither compressed nor uncompressed.

Read that again if necessary.

IEC 61937 allows compressed audio over S/PDIF. This could be any codec, but currently two basic ones are found in the wild: Dolby Digital and DTS. Dolby Digital allows up to 5.1 (discrete) channels and a bitrate of up to 640 kb/s. DTS allows up to 6.1 (discrete) channels and a bitrate of up to 1536 kb/s. There is no codec available today that supports 7.1 over S/PDIF. Newer codecs such as Dolby Digital Plus and DTS HRA do not support transmission over S/PDIF. When used for linear PCM, S/PDIF supports up to 96 kHz and 24 bits per sample. Anything transmitting 192/24 over S/PDIF is technically "overclocking" it, and while it can work, it is not part of the standard.

HDMI supports up to 8 channel linear PCM at up to 192 kHz and up to 24 bit per sample. It also supports the (concurrent) piggy-backing of a S/PDIF signal for compatibility with older equipment. HDMI Audio is not carried on separate wires in the HDMI cable; all of these signals are encoded into the main data stream during the vertical blanking interval. That is the time between the end of one frame of video and the beginning of the next (and is there for historical reasons because CRTs needed time to retrace the beam from the bottom to the top of the TV).

abraxalito pointed out that you could run 8 channels of 48 (or 44.1) kHz audio over an S/PDIF link running at 192 kHz. It just wouldn't be "S/PDIF" any longer. You would need to design a multiplexing scheme to insert a sample from each of the four (2-channel) streams in sequence, and extract them at the other end in the right order. This would not be easy.

I believe that you already found a good solution on eBay. It uses an HDMI input (remember you can't do 7.1 over S/PDIF) and gives you the 8 channels of analog audio. It's not expensive, and even without considering the investment in time, a DIY solution could not be made cheaper. You could still do some serious DIY mods to this thing. For starters, what kind of DACs and analog output stage does it use? Whatever it is, it probably isn't good enough for diyaudio folks. So mod it...
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Old 28th September 2010, 09:09 PM   #12
Xm0dgeek is offline Xm0dgeek  United States
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lol I didn't think of it as a bandwidth limitation... well that pretty much means I'll just make the receiver with a removable face plate and some headers to plug in the digital module when/if I ever find a solution.

Thanks for all the info so far, I'm not really partial to SPDIF but I did like it's interchangeable use of coax or optical but that really has nothing to do with SPDIF it self.

I was looking at a chip that would packetize up to 16 channels of audio data and send it over ethernet, but agiain I'd have the same problems with getting it to work like with the CS497024. As far as knowing what I'm doing and actually soldering it with out destroying it.

Now would sending 4 lines of digital audio over a say 25ft CAT6 ethernet cable using the 4 twisted pares be out of the question? After all I'm going for more of a one cable rather then a one wire. Of course 192Khz @ 24bits would be the preferable format but that's just wishful thinking perhaps?

HDMI would fit the 1 cable desire but from messing with HDMI audio before I'd rather avoid it, and I'd rather not loose an monitor for audio.

Thanks
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Old 29th September 2010, 12:24 AM   #13
wwenze is offline wwenze  Singapore
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Receiving 4 x stereo PCM SPDIF is ok (4 receiver chips) but you'll need to find a way for the source to send them, and through an ethernet connector to boot. Are you going to make your own source?

If you're building the receiver from scratch, why not go with ADAT instead.
Like: ADAT receiver

Also, how about a USB cable from your computer all the way to a box beside the amp, then 8 short cables from that box to the amp?

Last edited by wwenze; 29th September 2010 at 12:28 AM.
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Old 29th September 2010, 02:05 AM   #14
SoNic_real_one is offline SoNic_real_one  United States
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Denon Link III uses three SPDIF channels over three pairs of a CAT5E shielded cable (works with a regular one too). Also can stream three DSD channels. That means 3x2=6 audio channels...
The forth pair is the return channel (encryption negotiation?).
That's how I have connected my DVD-2930 player with the AVR-3805 receiver.
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Old 29th September 2010, 02:44 AM   #15
wakibaki is offline wakibaki  United Kingdom
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Quote:
Originally Posted by macboy View Post
You would need to design a multiplexing scheme to insert a sample from each of the four (2-channel) streams in sequence, and extract them at the other end in the right order. This would not be easy.
This is a doddle for any halfway competent digital engineer, but it is only part of the problem.

What is going to be the source of the 7.1 information? What software will deliver this information to the PC transmit interface? Where will the interface reside in the PC's I/O address space and what, if any, interrupts will it employ?

The problem is not one of creating an eight-channel protocol and receiver, this is comparatively trivial. The problem is rather one of how to originate and attach to the datastream at the transmit end. Once the 8 channels of LPCM are available in one form or another then they can be assembled onto some kind of high-speed serial data link, transmitted, demuxed and sent to 4 stereo DACs.

In the case of HDMI the data can be read from the disk by the bluray player and shot out the HDMI, whence it's recovery is comparatively straightforward.

Some PC mobos have HDMI, but whether the standard is fully implemented (i.e. whether it includes 8 channels of LPCM) is unclear. I don't have a bluray equipped HDMI PC, so I can't comment on that, other than to say that the bluray disks I have got have only got 5.1 sound and I only use the stereo RCA output on the bluray player.

w
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Old 29th September 2010, 04:06 AM   #16
Xm0dgeek is offline Xm0dgeek  United States
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Quote:
Originally Posted by wakibaki View Post
What is going to be the source of the 7.1 information?
w
Right now I've been living with a cheap 7.1 sound card form a local computer store parts bin. So I'd ether have to convert all the analog channels back to digital or find the data traced to the the DACs on the card.

but I've been eyeing a Asus Essence ST or a Xonar HDAV1.3 before that a HT | Omega Claro, but they havn't had any updates on that card for years now... I've seen some posts of taping into the IC2 before the DACs on the Asus cards.

If you're building the receiver from scratch, why not go with ADAT instead.

Quote:
Originally Posted by wwenze View Post
If you're building the receiver from scratch, why not go with ADAT instead.
I looked it up after rfbrw suggested it but it looked like it was older and not used much. But after reading that post about that ADAT decoder. A encoder in the PC sucking the data out of the sound card to the receiver over a optical cable sounds good to me
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Old 29th September 2010, 11:45 AM   #17
wakibaki is offline wakibaki  United Kingdom
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It occurred to me after I made the first post that we are needlessly complicating the issue. A USB recording interface with 8 channels will do what you want and appear in the Windows device list without any problems. I think it can be located up to 15 metres from the PC. You could even use a USB hub with 4 * 2-channel DACs.

What is the source of the program material (bluray, hard disk?), and what software do you currently use for playback?

w
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Old 29th September 2010, 02:09 PM   #18
qusp is offline qusp  Australia
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yes, the main issues is source material and format, pro audio has the answers for transport and we have the answers for the DACs
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Old 29th September 2010, 03:48 PM   #19
Xm0dgeek is offline Xm0dgeek  United States
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Quote:
Originally Posted by wakibaki View Post
It occurred to me after I made the first post that we are needlessly complicating the issue. A USB recording interface with 8 channels will do what you want and appear in the Windows device list without any problems. I think it can be located up to 15 metres from the PC. You could even use a USB hub with 4 * 2-channel DACs.

What is the source of the program material (bluray, hard disk?), and what software do you currently use for playback?

w
So use a hub to run a single USB cable to the receiver and split it to four, 2-channel recoding interfaces? or a single interface with 8 channels of playback?
How would Windows recognize the USB recording interface?
Like a generic sound device? or could I some how rought the sound from my soundcard to the USB interface? Kinda like what SRS AudioSandbox does with using the soundcard as a slave ( I think that's how to put it?)

Quote:
Originally Posted by qusp View Post
yes, the main issues is source material and format, pro audio has the answers for transport and we have the answers for the DACs
I'm not into crazy audiophile stuff, at least not yet. mainly I'll be playing DVDs that I've ripped to the HD (untill I get a blu-ray drive) I copy the whole VIDEO_TS folder so all the native formats should be there. I'll also be playing CDs ripped to FLAC, and of course MP3s from time to time. Then there's the matter of games, that's were I'll be getting the most use of the 7.1 for now. As for format I'm not exactly sure I guess 192Khz @ 24bit would be best then I could lower it if necessary.

But for now I have terribly mismatched speakers that would probably make you guys cringe, powered by 2 Sony XM-554ZR car amps with 8 RCA cables from the PC to amps. So I still have to make the speakers and rearrange my room, and before that I'll have to make a CNC Router to even do that ( the way I want too). So it's a longish term project.

Oh and yeah I'm powering the amps with my computer's 12V out, I'm just living with the noise till I get this stuff done so I can really compare night to day for sound quality
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Old 30th September 2010, 01:19 PM   #20
wwenze is offline wwenze  Singapore
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I was thinking of USB sound card with 7.1 channels of analogue output.
http://www.dealextreme.com/details.dx/sku.42470

I may be wrong about this, but Windows currently still does not allow one program to use more than 1 sound card at any one time, though if there would be somebody so kind (and pro) to help write a driver that emulates a sound card, takes the data and send to multiple sound cards simultaneously...

Kinda irritating when I want to A-B between 2 sound cards and I need to open two programs and play the music at the same time.

Last edited by wwenze; 30th September 2010 at 01:26 PM.
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