[mythtv-users] Max number of DVB channels per usb tuner

John Pilkington J.Pilk at tesco.net
Sat Dec 21 14:58:31 UTC 2013


On 21/12/13 14:22, Mike Perkins wrote:
> On 21/12/13 11:40, Mark Perkins wrote:
>> I was looking for something, don't remember what now, but came across
>> the following statement on a site advertised as a complete guide to
>> MythTV -
>> http://www.techradar.com/news/software/applications/complete-guide-to-mythtv-947946/4
>>
>>
>> ******
>> If you're using DVB (either terrestrial or satellite), you're in luck,
>> because these multiplex several channels into one stream. MythTV is able
>>   to receive an entire stream from a DVB tuner card, and then split it
>> into individual programmes.
>> When adding a capture card in mythtv-setup, press the Recorder Options
>> button to see a further screen, which includes a Max Recordings setting.
>>   Three is a sensible setting for this, and means that you could record
>> six channels at once if you have two capture cards.
>> ******
>>
>> Is that statement correct - ie that the number of channels that can be
>> tuned from a single multiplex is not a hardware "given" of the
>> particular card / tuner (in a DVB context)?
>>
>> I've set mine to two per usb tuner but that was because I thought I
>> remembered reading on the box (back when I had the box) that it was
>> capable of two channels.
>>
>> I've gone back to the manufacturer website to see what it says but it
>> only has the following statement which I am not sure whether it means
>> that it can tune two multiplexes (on a single usb tuner) and two
>> channels from each multiplex, or 4 channels from a single multiplex.
>>
>> ******
>> Multiple Video Processing mode, enables two TV tuner devices, up to four
>>   channel program processing simultaneously. You can simply upgrade your
>> TV enjoyments with one more WinFast digital TV tuner card/ box for
>> multiple TV viewing.
>> ******
>>
>> No doubt there are other considerations (ie hardware of the backend)
>> but I think USB 2.0 does 480Mbps and a multiplex is 20Mbps total so
>> the USB part should be fine. I think a 7200rpm HDD can do around
>> 60Mbps sustained write so that should be plenty as well.
>>
> Technically, the tuner is "tuning" a multiplex, since that is what
> happens to be transmitted on a particular frequency. Once tuned, the
> usual procedure is to filter out the one channel (multiplexed stream)
> the user wants from among those available.
>
> Of course, since the whole multiplex is available, and the filtering is
> done in software, it is (relatively) easy to select and process as many
> channels as the user wants from that multiplex. That's what 'multirec'
> in myth can do.
>

I've just remembered that I have two old USB tuners for which this might 
not be true.  They have hardware filters, and used to produce 'failed to 
open demuxer' messages in use, around once per second.

Now, for my 'emergency' third device, bought several years ago, I see:

  dmesg | grep dvb
[   10.662763] usbcore: registered new interface driver dvb_usb_af9015
[   10.663089] usb 1-3: dvb_usb_v2: found a 'TwinHan AzureWave 
AD-TU700(704J)' in warm state
[   10.880631] usb 1-3: dvb_usb_v2: will pass the complete MPEG2 
transport stream to the software demuxer

I use that device only when the PCI cards are already in use.  Project-X 
often reports packets out of order from it, and also blocks of this:

[202034.369192] usb 1-3: dvb_usb_af9015: rc query failed=-71
[202034.516313] usb 1-3: dvb_usb_v2: 2nd usb_bulk_msg() failed=-75

So... some are more equal than others.

John





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