[mythtv] Fwd: [linux-dvb] USB DVB-T tuners and MythTV

Adrian Wilkins adrian.wilkins at gmail.com
Thu Oct 6 13:39:41 UTC 2005


Reply from linuxtv list about MythTV and DVB-T problems that have been
observed with USB units. Any comments?

On 9/30/05, Patrick Boettcher <patrick.boettcher at desy.de> wrote:
> Hi Adrian,
>
> I read the ticket, but I never used MythTV so far, anyway, I think you are
> facing "normal" problems:
>
> 1) IMHO Myth should not rely on the signal-strength and
> signal-to-noise-ratio. The different hardware which is in use in the
> different devices are producing different results (because of calculation
> problems (not floats in kernel-space) and differences). For DVB/ATSC
> devices MythTV should rather monitor the LOCK-status, BER and UNC to form
> an opinion about the signal-quality. (For the nova-t-usb2 there is
> additionally a bug/mistake in pseudo-signal-calculation, which leads to a
> signal-strength of zero when the signal is actually _very_ good.)
>
> 2) USB1.1 devices (and USB2 devices running on a USB1.1 hub) that are
> using BULK transfers for MPEG2-TS transfer are facing a problem. The
> bandwidth of USB1.1 is not enough for receiving the complete transport
> stream of a DVB-T channel (which is about 12 to 16 MBit/s). That's why all
> DVB devices on USB1.1 using a PID filter in hardware to just get the
> packets with the requested PIDs.
>
> Problem:
> If only a PAT is requested it will take a quite long time to fill the
> BULK-transfer buffer. The BULK-URB won't return to the driver (and thus to
> the demux/section-filter) unless it is full. (That's why Malc has to use
> -5 as a parameter of dvbscan to get something (because the PID 0x10 (NIT:
> network information) is only repeated with a larger interval and it is
> small too)).
>
> Unfortunately I had problems by reducing the buffer-size of URBs (so that
> it won't take too long to get the URBs back)... which would be a solution.
>
> This problem does not exist for USB1.1 device using isochronous transfers
> for MPEG2-TS delivery.
>
> Patrick.
>
> --
>    Mail: patrick.boettcher at desy.de
>    WWW:  http://www.wi-bw.tfh-wildau.de/~pboettch/
>
>
> On Thu, 29 Sep 2005, Adrian Wilkins wrote:
> > A number of us are having problems with MythTV and DVB-T, mostly with
> > USB tuners, it seems.
> >
> > The longest thread on their bugtracker regarding this is
> > http://svn.mythtv.org/trac/ticket/343
> >
> > In particular, it's puzzling me that both my USB tuners, a Nova-T and
> > a DEC2000t, will record fine at the command line (using tzap), but
> > refuse to work with MythTV (they have previously worked just fine, it
> > seems to be the new signal monitoring code that Myth has introduced).
> >
> > The output from tzap looks weird though, esp. the Nova-T, which
> > reports zero signal as soon as it gets a proper lock (meh?). This is a
> > card that will stream a playable (mplayer) TS to disk with no trouble,
> > but Myth refuses to watch that channel.
> >
> > The problem would may be that Myth is waiting for PAT/PMT to appear
> > and considers it to be absent. The developers speculate that this may
> > be a driver problem.
> >
> > One assertation by a developer was that Myth probably gives the API a
> > bigger workout than your applications and that he might be able to
> > provide code that would break things ; I'm sure that would prove a
> > tasty challenge.
> >
> > I'm happy to perform whatever contortions necessary to aid resolving
> > this, as my wife is upset because House, MD hasn't recorded for four
> > consecutive weeks now and she's getting withdrawal symptoms ;-)
>


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