[linux-dvb] USB DVB-T tuners and MythTV

Patrick Boettcher patrick.boettcher at desy.de
Fri Sep 30 09:09:33 CEST 2005

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.

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.


   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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