Arduino Pro 3.3V/8Mhz, only 48*36 PAL works,

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What steps will reproduce the problem?
1. Using Arduino Pro 3.3V/8Mhz to generate the composite signal
2.
3.

What is the expected output? What do you see instead?
I have tried many resolutions but most of them had vertical synchronization 
problems.only 48*36 resolution worked

What version of the product are you using? On what operating system?
Arduino Pro 3.3V/8Mhz with TVOutBeta1 version with 680 Ohm Sync resistor and 
270 Ohm signal resistor

Please provide any additional information below.
I have written a detailed blog post at 
http://www.incrediblediy.com/2013/03/connecting-arduino-pro-33v8mhz-to-tv.html

Original issue reported on code.google.com by [email protected] on 20 Mar 2013 at 4:07