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[Hendrik]

I've been trying to setup TV using a DVB-C USB Tuner which I recently acquired. However, it fails to discover an entire channel group (for other German's reading, the ZDF group).
Its supposed to be on 450000 kHz, QAM256, 6900 Symbolrate.

I checked, and the profile I used includes this frequency (Germany.Kabel Deutschland (KDG)), but for some reason MC fails to identify the channels on this transponder.

A channel scan using DVBViewer discovered the channel as expected.

I manually added the channel to MC using the info discovered from DVBViewer, and it works fine then.

Is there anything I can do to debug why these specific channels would now show up in a scan?

[Yaobing]

Does it always not work for that particular frequency?  Have you tried re-scanning?

When scanning for channels, we basically go through entries in the profile one by one, submitting the tuning parameters and testing for signal lock.  If the signal does not lock, we move on, otherwise we wait for the "tuning changed" event.  Then we parse the channels.

So there can be two places this can go wrong, one is signal locking after submitting the tune request, the other is parsing.

[Hendrik]

Several attempts at re-scanning didn't reveal them. Using a slow scan also made no difference.
The log claims to set the frequency, and then a lot of waiting, until it eventually gives up, as it seems.

[Hendrik]

So on a crazy idea, I edited the transponder XML file and swapped the position of the 450000 QAM64 and QAM256 entries, so that the QAM256 entry would appear before the QAM64 one.
Crazy enough, that suddenly worked!

Just to confirm I wasn't going crazy, I reverted the change, did a re-scan, and the channels didn't show up anymore.

Maybe there is a glitch when switching transponders which have the same frequency, but different modulation?

[Yaobing]

Good find.  I will take a look there.

[Hendrik]

While I was in there, I noticed that the transponder list for Germany is quite old, so I'm going to submit an update with the latest channel assignments. Going through the infos now and assembling a list.

Edit:
Success, scan finds over twice as many channels now.
Now if I could just filter out the encrypted ones automatically somehow.....

[Yaobing]

Great, thank you!

I checked my code for switching transponders, and found no bug.  The list contain several other frequencies in similar situation with QAM64 and QAM256.  I wonder if any of those is also problematic.  I suppose we really should not have duplicate frequencies listed like that.  It should be either QAM64 or QAM256.  

This could be a device driver bug too.  We might end up in the situation if the device driver mistakes the new tune request as the old one because it has the same frequency.

[Hendrik]

The new list doesn't include any duplicates anymore.
I figure it came to be because different areas had the same frequency but with different modulations. But i cross-checked a wide variety of areas, and they are all using an identical frequency/modulation list now.

[Yaobing]

Now if I could just filter out the encrypted ones automatically somehow.....

I had planned to do that, but have not gotten around to it.  I need to figure out how to parse conditional access table for DVB.  For ATSC, the info is already parsed, and is in the Virtual Channel Table.