[mythtv-users] mythmusic iec958 help

[Mark Greenwood]

Hi Joe,

> Mark,
> 
> It (pardon the pun) sounds like your set-up is working great.
> 
> But the bulk of the issues you've seen here are related to an original 
> source sampled at 44.1KHz (CDRom) getting output thru the spdif into a 
> receiver/home theater amp/etc.  The problem is that many sound cards 
> (drivers) will re-code the signal as 48KHz.  This then causes the 
> resultant output from the receiver/amp to the speakers to be shifted up 
> in pitch about 10%.

I may be getting the various ongoing spdif threads confused here, but if your audio pitch is too high then your audio is not being re-coded anywhere. What's happening is that a 44.1KHz signal is being clocked at 48KHz - the samples are being played too fast, giving rise to the pitch change. The problem is that it *should* be being resampled to 48KHz somewhere along the line if the output sample rate is 48KHz. Resampling means exactly that - a 44.1KHz signal is re-coded using interpolation into a 48KHz signal, thus changing the sampling rate without altering the pitch. I may be saying the same thing as you in a different way, but I wanted to be clear where I'm coming from.

> In a previous post to a similar problem, I mentioned I have no 
> experience with the USB devices.  Your note sounds encouraging.  Please 
> let us know which Creative model you're using and what the manual says 
> about 44.1KHz spdif output.  If the USB "card" will output either 44.1 
> or 48 KHz (and sometimes 32 KHz) the manufacturer will usually brag 
> about it in the specs in the manual.  Please let us know; one of these 
> USB cards would be a good option for lots of people.

The "card" is a Creative Sound Blaster Connect. It has Stereo line in/out (analogue) and optical in/out. There's no selection of outputs - it simply sends the input signal to both the optical and analogue outputs. It does have a switch to set 'Digital Only' but the instructions don't say what that does and I haven't touched it. I've set it as the default card in my ALSA configuration. (Can't remember how, I used a command-line utility found by Googling). I have the optical out connected to my surround amp (a Yamaha DSP-E800). It doesn't mention anything about 44.1KHz spdif output. It cost me 30 quid, so I don't think it's anything special. The sound quality isn't the best, but for TV and the odd DVD it's fine. Does nicely for party music too :)

My myth settings are:
Audio output device : ALSA:default
Passthrough: Default
Max audio channels: 5.1
Upmix: Passive
AC3 and DTS passthrough enabled
Aggressive OFF
Internal volume controls ON
Mixer Alsa:Default
Mixer Controls: Master

FWIW the volume control doesn't work, but this doesn't matter as my surround amp has a volume control on it. I wouldn't expect to set the volume from my cable box or DVD player either.

I can play 44.1KHz mp3 files in MythMusic, and play the same track simultaneously on my iPod. After a bit of pausing and unpausing to get them in sync I can listen to one with one ear and one with the other. My ears are good enough to detect something playing at a different speed or pitch in that situation, and both stay perfectly in sync (and in tune) over 10 minutes.

I can use mythstream to listen to a 22KHz internet stream and that plays at the correct pitch. I'd definitely notice speech radio playing at double speed!

I can watch videos with soundtracks at any sampling rate and the sound stays in sync with the video.

I can watch DVDs and the amp says it's recieving a Dolby Digital input (if it's getting Stereo - e.g. TV - it says 'Pro Logic'), so passthrough must be working?

This is exactly how I would expect it to work. The sampling rate of the source material should be irrelevant, otherwise everything would have to be recorded at the same sampling rate everywhere and you wouldn't be able to play DVDs and CDs on the same machine. Which, basically, is the problem being discussed... I would not expect any digital sound system to be incapable of doing that. Something is very wrong somewhere if that isn't working.

Mark

> 
> Good luck,
> 
> Joe Henley
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