DIY 7.1 over Spdif? - diyAudio
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Old 27th September 2010, 07:05 PM   #1
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Default DIY 7.1 over Spdif?

Hello, I've been lurking here for awile but I can't seam it find what I'm looking for. I'm wanting to make a receiver that supports 7.1 over some kind of digital interface form a computer. Preferably something rather standard like SPDIF and it would be nice to hook a PS3 or something to it (don't have one). But mainly it's to eliminate the need to run 8 RCA cables from the back of my PC across my room into the amps and it would be nice for reduce noise when I install a carputer in the back of my SUV to the front were the head unit will be (can't have EMI over optical ).

Ive been looking at Cirrus Logic's CS8416 but I'm rather unclear how I would get 8 channels out of it to run to a DAC. In the CS438A's evaluation board they use a CS8416 to get 7.1 but it looks more like 6 channels and just fill in the rear 2. But that's mainly based off there schematic.

I'm planing on controling this with one or more arduino based microcontrollers. I've been working on a sub amp for a friend that can be controlled with his AVR remote. stuff like volume and crossover frequency and controling the room lighting .
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Old 27th September 2010, 07:28 PM   #2
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In short, you can't, unless it is compressed. Then you have to decompress it to get the individual channels.
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Old 27th September 2010, 08:54 PM   #3
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Ah yes that's what I was afraid of, and to uncompressed it you have to have access to proprietarycodeics to decompress them right? What about LPCM? I think that's royalty free? and it should support 8 channels right?

How would I go about decompressing the spdif signal then?
Cirrus has this part that seams to fit the bill and have lots of other stuff to play with too.
CS49700 Family : 32-bit High Definition Audio Decoder DSP Family
Mouser has them for $36.56 for one.
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Old 27th September 2010, 11:50 PM   #4
wwenze is offline wwenze  Singapore
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Using a card that can route the uncompressed 7.1 into 4 x 2-channel PCM on 4 SPDIF cables, yes.

SPDIF supports only 2-channel for LPCM.
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Old 28th September 2010, 12:09 AM   #5
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Quote:
Originally Posted by Xm0dgeek View Post
Ive been looking at Cirrus Logic's CS8416 but I'm rather unclear how I would get 8 channels out of it to run to a DAC.
If your original channels are 44k1 or 48k, 16bit, you could achieve this with no compression if you ran the SPDIF link at 192kHz. You'd have bandwidth to spare then too as the link supports 24bits. Still a fair bit of logic to design to multiplex them at the input and demux at the output, but could be done.
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Old 28th September 2010, 03:10 AM   #6
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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?

The more I look at it I'm likeing that CS497024 more and more. Has anyone had any experience with it? Soldering it will be quite the challenge I think.
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Old 28th September 2010, 10:41 AM   #7
wwenze is offline wwenze  Singapore
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Just that HDMI has enough bandwidth for 8 channels of LPCM, audio data is sent on the same pins as video, in a format different from SPDIF.

SPDIF was designed for 2 channel PCM, but it's just another interface which you can send anything on. DTS and Dolby encode multi-channel audio such that they can be sent over SPDIF, but without a decoder if you feed this to a stereo SPDIF receiver straight you get just noise.
An analogy would be, open a text file with notepad, you see text, open it with paint, you see garbage. But a .bmp renamed to .txt opened with paint...

It can't be too bad sounding if movie companies used this to get 5.1 out of the DVD player.

Last edited by wwenze; 28th September 2010 at 10:44 AM.
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Old 28th September 2010, 11:23 AM   #8
qusp is offline qusp  Australia
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hehe you will find that the chip is just the first thing you have to buy t make that chip work. you will need to be able to program an MCU, probably apply for a licensee to decode the codecs, pay for a high level PCB design, pay for placement of the chip on this PCB (good luck doing that by hand) then you will have to find a way of sending it the data in the first place
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Old 28th September 2010, 12:07 PM   #9
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Quote:
Originally Posted by Xm0dgeek View Post
Ah yes that's what I was afraid of, and to uncompressed it you have to have access to proprietarycodeics to decompress them right? What about LPCM? I think that's royalty free? and it should support 8 channels right?
SPDIF won't support 8 uncompressed channels over a single link but ADAT will.
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Old 28th September 2010, 12:58 PM   #10
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Quote:
Originally Posted by Xm0dgeek View Post

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?
HDMI - Wikipedia, the free encyclopedia - that's the simple version - if you wanted to look into designing something then it's off to the chip makers and some engineering design info.....................

For the price of that unit I don't think you will be able to DIY a cheaper or better unit. If you want to spend a few thousand $$$$ and a bunch of time designing a PWB and software you could get something going - but what's the point in that?
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