[mythtv] Fwd: iec958 output (not passthrough) with Alsa rate pluginproblem

Yeechang Lee ylee at pobox.com
Tue Jan 16 17:40:21 UTC 2007


John P Poet <jppoet at gmail.com> says:
> Not all sound cards require 48KHz.  For example, sound cards based
> on the Envy24 chip will pass through 44.1KHz just fine.

I have a different problem in not being able to play 32kHz audio from
recorded programs over optical. As I wrote at
<URL:http://www.gossamer-threads.com/lists/mythtv/users/241814#241814>,
with some followup comments interspersed:

    A much more serious issue is 32K AC3 output. As Rob Baumstark and
    others report
    (<URL:http://www.gossamer-threads.com/lists/mythtv/users/207600#207600>),
    certain FireWire recordings off certain basic-cable channels come
    with AC3 audio tracks in 32khz, which give my new receiver fits
    (even to the point of shutting itself down to, I presume, avoid
    physical damage to the speakers).

(NB: Upon reflection, I don't know if the issue actually lies with the
receiver not being able to hand 32kHz or if if it's my Turtle Beach
Riviera not being able to properly pass through 32kHz audio over the
optical connection. I note that the receiver's front LCD displays
"Dolby 2/0" at such times, which makes sense.)

    It's not consistent; one recording from TCM, for example, is fine
    while another isn't.

(An indication, to me, that this is indeed some kind of bug in
mythfrontend; thus <URL:http://svn.mythtv.org/trac/ticket/2813>.)

    The sample .asoundrc found at the Wiki didn't help, unfortunately;
    "ALSA:mixed-digital" with the .asoundrc produced the same results
    as using no .asoundrc at all.

(Said .asoundrc, by the way, has me--and, it sounds like, others as
well--totally stumped in its baroqueness. Of course, it doesn't help
that the ALSA developers intentionally don't provide much enduser
documentation; I saw an exchange on an ALSA mailing list last night
while fiddling with this issue that absolutely astounded me. In any
case, since my setup works fine otherwise without any .asoundrc at
all, I'd like to keep things that way unless Lawrence's writeup or
some other solution requires otherwise.)

    I've been working around the issue by taking advantage of MythTV's
    automatic software remixing of AC3 tracks at non-100% speeds. I
    created a new Playback Group in which I specify a playback speed of
    105%. I then set affected recording rules (Futurama off The Cartoon
    Network, for example) to use it. This, of course, isn't ideal: a)
    While a 105% playback speed is almost indistinguishable from the norm,
    it's still an annoyance. b) I can't make ex post facto changes to the
    recording rule for single recordings.

The above raises an obvious and easy-to-implement (at least to this
naive non-coder's eyes) bugfix for the issue: Have mythfrontend
automatically remix 32kHz audio to 48kHz (which, along with 44.1kHz,
my setup can play with no issues) even at 100% playback speed. Yes,
this would theoretically mean I'd be losing the Dolby, but given that
this is 2.0 audio I'd really be giving up nothing. And given the
number of setups that have trouble with 44.1kHz I'd be surprised if
they could play 32kHz by contrast.

-- 
Yeechang Lee <ylee at pobox.com> | +1 650 776 7763 | San Francisco CA US


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