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[linux-dvb] Re: [rfc/patch] pll handling and cx22702 update
From: "Andrew de Quincey" <email@example.com>
> The TDM1316 datasheet has different recommendations for when you are
> scanning for channels than when you are locking onto a channel for
> receiving data for display. The scanning mode "enables a fast tuning
> speed". I'm not actually sure _why_ it is better to disable fast tuning
> speed when locking for receiving data for display - it just says that
> it is. The above was an attempt to expose this as a starting point
> for higher level support. I have seen this sort of thing in a couple
> of other datasheets.
I wonder, though, if this applies to digital tuning - keep in mind that the
TD13xx is a hybrid tuner for analog and digital applications. Have you ever
seen something like this in a datasheet for a digital-only tuner/PLL...?
Analog tuners often have different PLL charge pump setting recommendations
for searching and viewing. From what I understand, the "searching" setting
will cause th PLL to lock quicker, at the expense of "phase noise", i.e.
there'll some a bit of "frequency jitter" in the output you get. This may
be acceptable with an analog TV signal, where the sync signals should be
easy enough to detect even in a "noisy" signal. But how well can a DVB
demodulator cope with a signal with frequency jitter? It might just as well
_slow down_ the locking process of the demodulator, i.e. slowing down the
locking/searching instead of speeding it up...
Maybe it would be a good idea to run some tests, measuring the signal
presence detection and locking times of the demodulator after tuning with
"normal" and "search" charge pump settings. If you find the "search" charge
pump setting doesn't _significantly_ speed up the signal detection/locking,
I'd say it's not worth implementing this mode...
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