[mythtv-users] Summery of my problems

Rudy Zijlstra mythtv at edsons.demon.nl
Sun Jan 29 01:29:28 UTC 2006

Michael T. Dean wrote:

> 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).

probably good to improve, but....

for the silicon tuners i know about, (which are both the ones we use on 
our products as well as some others), this makes no difference at all. 
Most modern (silicon) tuners are able to finetune to the correct 
frequency if the initial frequency is within 125KHz of the correct 
We make use of this is the scanning algo, as this makes it possible in 
Europe to scan with 250KHz steps in case we have to do a frequency sweep....
Please keep in mind, Europe cable does not have any standard to adhere to.



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