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Old 19th December 2011, 03:18 AM   #1
RHosch is offline RHosch  United States
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Default HDMI transmitter: diy, eval board, finished product?

Thinking about what is required to convert multichannel pcm likely at i2s level into an HDMI stream. Standard HDMI transmitter should suffice, but for the life of me I can't find anything in any stage of finished form available for consumers. I thought I'd surely find a piece of professional studio gear or some conversion box that takes multichannel digital audio inputs (probably in spdif form) and outputs as HDMI carrying the multichannel lpcm stream. No such luck, unless someone can point me in that direction. The only thing that might work is sequential conversion boxes that may take multiple spdif inputs and output SDI followed by SDI to HDMI conversion, though the details of those products arent clear and I'm not sure if multichannel audio is actually supported and whether that is even combined into the HDMI stream.

Looks like a number of HDMI transmitter chips are available, but haven't seen any DIY forms or PCB designs either. Haven't looked into evaluation boards yet because of NDA and I'm really just at the curiosity level.

Anyone have any experience with this? Know of any products or projects that do this?

For context, I'm considering tapping multichannel i2s outputs from one Avr/preamp prior to dacs and providing as input to a second preamp/avr for further processing. The only way to feed multichannel uncompressed audio in consumer level gear that I'm aware of is hdmi, thus the purpose of this thread. Pro gear doesn't offer the same processing, and I doubt outputting Madi format would be easier anyway.

Thoughts?
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Old 20th December 2011, 05:35 AM   #2
RHosch is offline RHosch  United States
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Wow, I really am that crazy!

The block diagram for say an analog devices ad8998b looks simple enough. Cheap chips in quantity, haven't asked for a small count quote from any supplier. Evaluation boards for similar chips run around two grand and eventually this crazy idea would call for two, so not very attractive.

But even if I could get the chips and understood the implementation, a PCB design for bga or surface mount package working at these frequencies is beyond my comfort level. Way beyond.

Has no one seen a product or project like this? Am I the only one who has dreamed up a need for something like this? No one else wondering if they can add dsx sides to logic 7, or expand a pair of height channels with dplIIx, or... and keep everything in the digital domain...
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Old 20th December 2011, 08:56 PM   #3
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Just bypass the HDMI rx on the second device using whatever method you find simplest be it ribbon cable or a few lengths of coax.
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Old 21st December 2011, 02:42 AM   #4
RHosch is offline RHosch  United States
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Hmm.. that's an interesting thought. I have been concentrating on getting digital signals out, not thinking much about how to get them in. Not that familiar with HDMI rx in general, would have to see if asrc was handled by the HDMI rx and whether I needed one between i2s out and in.

Thanks for the suggestion. I had completely overlooked that possibility.
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Old 21st December 2011, 06:00 PM   #5
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Why, in the name of all that is digital, would an ASRC rear it's ugly head in a HDMI receiver ?
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Old 22nd December 2011, 05:28 AM   #6
RHosch is offline RHosch  United States
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Dunno... said I wasn't very familiar with HDMI tx/rx. I guess what I was really wondering is whether there might be issues of compatibility between i2s signals pre-dacs on device one and expected i2s signal post-hdmi on device two. I understand that word length shouldn't matter as msb is sent first, but wasn't sure about clock rate, which device would try to act as master, etc. As i read more about i2s signaling, it seems that it isn't quite as simple as "i2s always = i2s". For spdif signaling it seems an asrc is typically employed to render the connection rather idiot proof and allow various source and receiver devices to work despite sample/bit differences. Was wondering out loud what issues I might have going that route and whether an asrc would play any role.

Seems there's also the issue of how far I'd need to send the signals and whether passive i2s would be possible or whether I'd need to extend the range either by lvds or spdif (and I'm not sure which would be worse - seems each has its issues).

I'm.sure all of this is difficult to approach in the abstract. Probably easier if/when I actually have pieces of equipment in hand and can ask questions about a specific application. But your response at least gave me hope that this is possible, so thanks for that.
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Old 22nd December 2011, 05:02 PM   #7
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A transmission medium that irrevocably alters that which it transports tends to be frowned upon. ASRC's are not a required component of HDMI or SPDIF transmission.
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Old 24th December 2011, 06:38 PM   #8
ashaw is offline ashaw  Australia
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Yes ASRC is not required, however you do have to re-clock the signal in HDMI as the audio bit-clock is completely lost in the transmission, the data is transmitted in the inter-frame blanking time between video frames, Good use of wasted bandwidth but not kind to jitter rates....
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Old 25th December 2011, 05:50 AM   #9
RHosch is offline RHosch  United States
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Ok, well no concerns there if the HDMI receiver is bypassed. Seems I would face a few early challenges which might depend on the specific products chosen for modification. One is whether the i2s outputs on device 1 were at the same sampling rate as the expected input on device two, whether that output changed based on program material/source or was always the same post DSP engine... basically making sure the two devices could speak the same digital language. The second that comes to mind is handling the clocks. Each channel on device 1 could vary a bit if it was just headed for a dac. Heading in to another device, I wonder if all clocks would need to be referenced to one, whether device 1 or 2 should be master, and how hard that would be to accomplish. No idea here. The last thing is transmission distance, whether I could get the devices close enough for i2s connections or whether I'd have to go through spdif or lvds and if so which would be better.

Again, questions probably easier to tackle with devices in hand, though i do wonder if there might be certain intrinsic properties of certain devices re input and output digital stages that might make some more amenable candates to experiment with than others.
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Old 25th December 2011, 06:49 AM   #10
dangus is offline dangus  Canada
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A PC can accept S/PDIF inputs, and send sound out over HDMI. Exactly what happens in the middle, I'm not sure. Mysterious DirectShow graphs?
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