[mythtv-users] My experience with S/PDIF, 32khz AC3, and Playback Groups regexps

Yeechang Lee ylee at pobox.com
Sun Dec 17 20:50:11 UTC 2006

(This is part of an occasional series on how I've successfully [or
unsuccessfully] accomplished something in MythTV on my Fedora Core
6-based frontend/backend system using the ATrpms packages. I will
assume that readers can do basic Linux tasks, such as install RPMs,
edit /etc files, and generate xorg.conf. I hope to cover the mystery
areas where others may most often fall astray in.)

I spent part of this weekend setting up my new surround-sound system
(An Onkyo HT-S790 from Crutchfield. $420 with free shipping and no
sales tax!). I'd already bought ahead of time an optical cable from
Monoprice and--as recommended by many here--a Turtle Beach Riviera for
its optical out jack, and have been enjoying 5.1 goodness (I haven't
bothered to set up the two extra surround speakers and may never
bother) from my MythTV setup. I still use neither /etc/asound.conf OR
.asoundrc; I simply had to turn IEC958 Output on in alsamixer.

Willy Boyd <willyboyd at gmail.com> wrote the following two months ago,
when discussing the Turtle Beach Audio Advantage, a USB device:
> FWIW I turned off "extra audio buffering" in the frontend and that
> helped out the delay [in audio when skipping] I was seeing quite a
> bit.  No other issues so far.

I found that I had to keep this setting on, not off. When playing back
ABC's broadcast tonight of Finding Nemo, I found that turning this
setting off led to "Writeaudio: buffer underrun" messages, with audio
dropping in and out. The receiver was also obviously getting confused
by the audio signals it was receiving (and not receiving).

A much more serious issue is 32K AC3 output. As Rob Baumstark and
others report
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). It's not consistent; one recording from TCM, for
example, is fine while another isn't. 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.

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.

For b), the regexp feature of Playback Groups is an possible solution,
in that I could manually specify affected programs' names . . . Except
I can't get regexp to work at all here, something I'd previously
noticed. For example, even after creating a new Playback Group called
Football, and putting


in the regexp field for the group, existing recordings titled "College
Football" still play back with the Default settings and new recording
rules don't automatically pick up the Playback Group setting. Again, I
could manually specify the Playback Group within recording rules, but
then what's the point of claiming to offer regexp?

I'll go ahead and file a ticket for the 32khz issue since there isn't
one already. For the Playback Groups regexps, I am surely missing
something really obvious, but what?

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

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