Author Topic: Using one crystal for both PAL and NTSC  (Read 1389 times)

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Offline c64Topic starter

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Using one crystal for both PAL and NTSC
« on: May 10, 2020, 04:16:12 am »
I'm looking at datasheets for few video decoders (for example ti TVP5150)

They all use 14.31818 (or 28.63636) MHz crystal to capture both PAL and NTSC. I guess it's choosen because it's exactly 4x(8x) of the NTSC colorburst. But it's not any fraction of the PAL colorburst frequency. So how does it work for PAL?

NTSC colorburst is 3.5795454 MHz
PAL colorburst is 4.4336187 MHz
 

Offline Renate

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Re: Using one crystal for both PAL and NTSC
« Reply #1 on: May 10, 2020, 01:53:04 pm »
That's an interesting question and I don't know.

NTSC = 4.5 MHz (audio subcarrier) / 286 (harmonic of line freq that hits audio) / 227.5 (color cycles in line)
NTSC = 2^3 * 3^2 * 5^7 *7 / 11 Hz

PAL = 25 Hz (frame rate) * 625 (lines) * 283.75 (color cycles in line) + 25 Hz (offset)

If we ignore that pesky 25 Hz offset

PAL = 5^7 * 227 /4 Hz

So the ratio between the two is:

2^5 * 3^2 * 7 / (11 * 227)

2016 / 2497

So, in short, I don't know.


 

Offline Bud

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Re: Using one crystal for both PAL and NTSC
« Reply #2 on: May 10, 2020, 03:00:42 pm »
Likely it is done using a PLL ( phase locked loop). You have two oscillators, one is the reference, the other one adjustable (voltage controlled). You divide outputs of both oscillators to arrive to a same common frequency, then feed these outputs to a phase frequency detector. if both frequencies are the same the ouyput of the detector is neutral. If they are not, the detector will produce a positive or negative voltage which is fed back to the adjustable oscillator and adjusts its frequency up or down to equalize the divided frequencies at the detector input. This way you can have two or more oscillators running at different frequencies derived from a common reference, as long as a common PFD frequency can be found for the dividers.
Have a read on PLL theory of operation to understand this better. PLLs are widely used in electronics so at some point you will run into it anyways.
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