[mythtv-users] Summery of my problems

Michael T. Dean mtdean at thirdcontact.com
Sat Jan 28 20:44:41 UTC 2006


On 01/28/2006 12:46 PM, Dylan R. Semler wrote:
> Michael T. Dean wrote:
>> On 01/27/2006 01:07 AM, Robert Johnston wrote:
>>> On 1/26/06, Dylan R. Semler <dsemler at macalester.edu> wrote:
>>>> For analog, the picture quality is pretty bad (noisy)...  The picture quality is much better using tvtime and those lines do not appear.
>>> As for the noise, make sure that Myth is tuning into the same
>>> frequency (EXACTLY the same frequency) as TVTime is. It sounds like
>>> Myth's off by a few Khz.
>> Being off by a few KHz shouldn't matter.  Many of the NTSC cable channel 
>> frequencies are off by 12.5 KHz (and the same goes for IRC, and HRC 
>> varies by 5-15KHz), but the tuner hardware's fine-tuning mechanism 
>> should still be able to find the appropriate center frequency.  See 
>> http://www.gossamer-threads.com/lists/mythtv/dev/164614#164614 (which 
>> was totally ignored by the guy complaining in Trac that the frequencies 
>> are off) for an in-depth listing of what frequencies are off and why it 
>> shouldn't matter.
>>
>> If you (Dylan) want to try an academically-correct list of frequency 
>> definitions, I'd be happy to create one for you (against SVN--just let 
>> me know which frequency list you're using: ntsc_bcast, ntsc_cable, 
>> ntsc_hrc, ntsc_irc (I'm assuming NTSC based on your e-mail address)), 
>> but you'll have to do all the testing because I don't have any 
>> RF-modulated sources.
> Well it's worth a shot.  I'd love to get better picture quality.  I'm 
> using ntsc_cable.

OK.  Attached is a patch that will correct the frequencies used for 
ntsc_cable, ntsc_hrc, and ntsc_irc.  Those used for ntsc_bcast are 
already correct.  Apply the patch to current SVN and recompile and 
install to test.

Note, however, that it may be better to leave ntsc_cable (standard 
cable) unmodified.  The modified channels in the standard cable 
definition are definitely using frequencies that are not used by the 
cable company, but that's because the FCC requires that they be offset.  
(Cable channels operating on frequencies used in the aeronautical 
radiocommunications bands 118-137MHz, 225-328.6MHz, and 335.4-400MHz are 
required to be offset by 12.5kHz from 25kHz-spaced channels, and those 
on frequencies used in the aeronautical radiocommunications bands 
108-118MHz and 328.6-335.4MHz are required to be offset by 25kHz from 
50kHz-spaced channels.)  The FCC doesn't specify whether the offset 
should be positive or negative, but it's more common within the cable 
industry to use a positive offset.  The current standard cable 
definition uses the frequency before the offset (which is right in the 
middle of a positive or negative offset), but the patched version 
assumes a positive offset (meaning that if a cable company uses a 
negative offset, we'll be off by 25kHz on many channels and 50kHz on 98 
and 99).

So, since you're a standard cable user, please let me know if you really 
do see a difference with the modified frequency values--and try not to 
be swayed by the placebo effect.  ;)  If there really is a difference 
(and I still don't think there will be), we may want to call the 
modified ntsc_cable "NTSC Cable (1)" and create another that uses a 
negative offset and label it "NTSC Cable (2)".

Those most likely to see any difference with the patch are HRC users (as 
some of their frequencies--mainly the higher frequencies--were off by as 
much as 60kHz).  However, it's likely the only difference they will see 
will be a slightly faster tuning cycle (unlikely to be noticeable in 
Myth, though, because of the other "delays" in channel changing--and, 
quite possibly, not noticeable at all by human perception even if you 
tried the before/after frequencies in tvtime or something).

Also, technically, standard cable doesn't have a channel 1, and HRC and 
IRC don't have T-7 through T-14 (but, I "fixed" the IRC definitions, 
since IRC was a modified copy of standard and standard had defined T-7 
through T-14 incorrectly).  While we could remove them, it's probably 
not worth the effort...

BTW, this patch definitely needs testing before being applied to SVN 
(and should probably do something smarter with the standard cable 
definition, as described above).  Therefore, I'm not putting it in a 
ticket, but it's here for anyone who wants to test/use it.

Mike
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