some audio questions regarding SPDIF interface

[saint-francis]

I have recently changed to SPDIF from analog on my HTPC. I am encountering some issues.

First of all, my receiver decides randomly to not accept the signal from my remote (It’s a POS). I have found that when using SPDIF software volume controls don’t work. MPC HC, My audio driver and Windows seem to have no effect on volume. Is this something I’m doing wrong or is this just how SPDIF works?

Second, I am in serious need of an equalizer. My receiver doesn’t have one. Do software EQ’s work with SPDIF? Are there hardware EQ’s that work with SPDIF?

The reason I’m using SPDIF is that the analog cables I have were introducing some speaker noise. When the speakers were on and there was no music or a movie playing they produced a hiss. Also SPDIF lends itself to better cable management since it’s one cable instead of three. If I don’t select SPDIF in FFDShow will it decode the audio and send it properly over the coax cable and still give 5.1 sound? I don’t think I really need to decode on the receiver. The HTPC sould be fine.

Thanks

[Blue_MiSfit]

The only real problem with S/PDIF is that it only has sufficient bandwidth to transfer 1.5mbps - either DTS or Dolby Digital. So for 5.1 sources, if you do the decoding on your PC and do some processing (i.e. EQ / whatever), you can't send the uncompressed result out to the receiver. You have to re-compress it to Dolby Digital or DTS again :(. 2ch audio is perfectly fine, since S/PDIF's bandwidth is sufficient to carry it uncompressed.

HDMI solves this, by having sufficient bandwidth to send at least 8 channels of 24 bit 96 KHz uncompressed audio, as does AES/EBU - the professional variant of S/PDIF

Sadly, if you must apply EQ on a 5.1 source, and you must do it on your PC, you're gonna end up recompressing it. My suggestion is to use ac3filter, since it can both do EQ and re-encode to 640kbps Dolby Digital on the fly very well. It's also just a darned cool piece of software!

Personally, I don't care about 5.1, and downmix everything to 2ch anyway - so S/PDIF is perfect for me :D It feeds my receiver through a skinny little RCA cable (ROFLcopter at the overpriced "coaxial digital audio" cables they sell), no problems.

~MiSfit

[saint-francis]

Can FFDShow do these things? And are there any hardware EQ's that work with SPDIF? I'm assuming not since Google doesn't turn up too much and I assume that the hardware would need to do the de encoding/re encoding.
Thanks for the response.

[TinTime]

FFDShow's got an AC3 encoder in it's output section so you should be able to use it. Reclock will do AC3 encoding too.

Another alternative would be to get a soundcard with DD Live and / or DTS Interactive. Obviously this would cost money, and (IMO) be of little benefit over the previously mentioned AC3 encoders, unless you want to send multichannel over S/PDIF where directshow isn't an option.

If you want a hardware solution, I think that would simply be a new amp / receiver, given that it would need to have a built in DD / DTS decoder. What you're asking for is really a home cinema pre-amp which I think is going to be pricey!

As an aside to Blue_MiSfit, isn't it a standards issue with S/PDIF, rather than bandwidth? It's annoyed me for a while now that S/PDIF will carry 192Khz/24bit stereo PCM but there's no standard support for, say, 48Khz/24bit 7.1 PCM. Or 5.1 even.

[saint-francis]

Thanks for the answers guys. I'm about to wire my room right now. I'm putting in network drops throughout the house and I figured while I'm at it I'll put in drops for the SPDIF between the HTPC and the receiver. I've already got the holes cut and drilled and I have cable and BNC connectors for the SPDIF drops. I still have time to go out and get some good shielded stereo cable if I want to go analog instead/also. I don't want the limitations of SPDIF but I don't want the noise I've had recently with analog either. :(

[Blue_MiSfit]

If you're doing some wiring - why not wire for HDMI?

~MiSfit

[saint-francis]

My receiver doesn't take it. And I'm not running conduit. I'm putting jacks in the wall under my HTPC which run to the wall behind my receiver. Are there jacks for HDMI that can be mounted in a wall plate? Can HDMI cables be made? I have no first hand experience with HDMI. Furthermore, I thinking I'm not going to go with SPDIF. It's a plain PITA. Way to many limitations. The jury is still out. I put off my construction until next week while I experiment with SPDIF more. Now I'm going to have these gaping holes in the wall until next Saturday. :rolleyes:

[Blue_MiSfit]

:D The wife / other will love that!!!

SPDIF is handy-dandy, but it's a real bummer it was never revised to support higher bandwidth applications. A toslink fiber cable can definitely carry more than 1.5mbps ;)

If you can manage, well-shielded analog might be your best solution.

I don't think it's trivial to _make_ HDMI cables, but you can certainly do in-wall installation. Monoprice sells good cheap cables.

~MiSfit

[saint-francis]

Well I have a spool of shielded fireproof cable that is very good. I might use it. Thanks for the input.

Can anyone help me set up FFDShow for proper use with SPDIF? I have a 5.1 system.

http://i182.photobucket.com/albums/x270/saint-francis-photos/Untitled-5.jpg

http://i182.photobucket.com/albums/x270/saint-francis-photos/Untitled2.jpg

Will these settings allow me to use the equalizer? As I understand it I need to decode the audio, run it through the EQ and then re encode it to send it on it's way to the receiver. Sounds pretty convoluted to me.

Are there any more sophisticated eq's I can use with either SPDIF or analog?

What does "encode only 5.1 ch streams" mean? Does this mean that it will only process 5.1 streams, or does it mean that it will encode any streams it is fed to 5.1?

Is setting "output speaker configuration" to "same as input" the proper thing to do or should I set it to "3/2/1 6 channel"? What is the difference?

Will the "expand stereo to center" and "expand stereo to surround" options work?

Tank you for you time.

[TinTime]

As I understand it I need to decode the audio, run it through the EQ and then re encode it to send it on it's way to the receiver. Sounds pretty convoluted to me.

That's exactly what you're trying to do. It's not that convoluted (IMO at least :)) - it's exactly the same as when you use analogue outputs, but with an AC3 encoder on the end. In fact it's probably simpler as you don't need to expand 2.0ch to 5.1ch, assuming that your amp will apply some kind of DSP to expand all AC3 input.

Just keep the equalizer filter. You shouldn't need the mixer.

Also tick the "Encode only 5.1ch streams" box. Stereo PCM can be passed over S/PDIF as is, so this will prevent it from being encoded back to AC3.

Not sure about other EQ options available to you. I don't use these myself.

NB once the audio is encoded to AC3 you won't be able to change the volume. Your soundcard volume controls still won't work. You'll need to use the volume filter in ffdshow, which isn't exactly ideal.

[saint-francis]

Just keep the equalizer filter. You shouldn't need the mixer.

So just leave it on "the same as input?

Also tick the "Encode only 5.1ch streams" box. Stereo PCM can be passed over S/PDIF as is, so this will prevent it from being encoded back to AC3.


I'm not quite following you. What if I have a stereo MP3 stream? Will that be encoded and passed along and encoded to the receiver?

NB once the audio is encoded to AC3 you won't be able to change the volume. Your soundcard volume controls still won't work. You'll need to use the volume filter in ffdshow, which isn't exactly ideal.

Right. One of the limitations of SPDIF.

[TinTime]

So just leave it on "the same as input?

I meant just don't have the mixer active.

I'm not quite following you. What if I have a stereo MP3 stream? Will that be encoded and passed along and encoded to the receiver?

No, stereo MP3 will be decoded to PCM and then passed out of the S/PDIF.

[leeperry]

HDMI solves this, by having sufficient bandwidth to send at least 8 channels of 24 bit 96 KHz uncompressed audio
apparently the "maximum supported audio stream bandwidth is 6.144 Mbps" on the ATi :
http://ati.amd.com/products/radeonhd4800/specs.html

16-bit/48 LPCM 5.1 is 4608 Kbps
24-bit/48 LPCM 5.1 is 6912 Kbps
16-bit/48 LPCM 7.1 is 6144 Kbps
24-bit/48 LPCM 7.1 is 9216 Kbps
24-bit/96 LPCM 5.1 is 13824 Kbps

so "8 channels of 24 bit 96 KHz uncompressed audio" over HDMI is not gonna cut it, I think..