NTSC Software Decoding Presentation
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Transcript of NTSC Software Decoding Presentation
![Page 1: NTSC Software Decoding Presentation](https://reader034.fdocuments.net/reader034/viewer/2022050808/5558b9f4d8b42a7e298b4ee5/html5/thumbnails/1.jpg)
Software Based NTSC Signal Processing
Prateek Mohan Dayal
Dr. Debashis Ghosh
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Organisation Of The Presentation
• Problem Description
• Discussion of Decoding Algorithms
• Conclusions
• Directions for Future Work
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Problem Description
• Demodulate NTSC Over The Air (OTA) broadcast Signal to get sound and video
• Audio decoding completed last semester
• Video decoding implemented this semster
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Structure of the Samples
• Complex Samples @ 8 Msamples/sec
• 16 bit real and 16 bit imaginary part
• Video carrier at -1.75 MHz and Audio Carrier at 2.75 MHz
• Instantaneous Phase and Envelope Information can be very simply calculated from these samples
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FFT of Samples
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Video Decoding Algorithm
• Interpolation– Interpolation prevents alisaing when bringing
the video down to 0 Hz– We interpolate the samples to 24 Mhz– The effectiveness of this step depends on the
interpolating filter’s high freq. response
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Interpolating Filter Response
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Algorithm contd…
• Supress the audio carrier
• Shift the video carrier to baseband
• Decimate a copy of these samples– The decimated stream is used to extract
timing information – This reduces the computation for timing
calculation– Decimation by a factor of 3
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Algorithm contd…
• Sync Separation– Using a comparison with blanking level
• Sync Processing – Using a cross correlation based approach– The correlation peak locates the sync– Vertical/Horizontal syncs are differentiated
based on their duration– Jitter can be calculated based on correlation
output length
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Vertical Sync Detection Sequence
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Vertical Sync Search Result
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Horizontal Sync Detection Sequence
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Horizontal Sync Search Result
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Algorithm contd…
• Even/Odd Field Detection– Using the distance of the last horizontal pulse
from the first pre-equalizing pulses– Field is Even if this distance is H/2– Field is Odd if this distance is H
• Skip blanked lines– First 21 lines for Odd Field– First 18 lines for Even Field– About 400 active lines of video
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Algorithm contd…
• Build Frame – For every line
• Locate horizontal sync• Calculate jitter• Using the above information collect sample for
each of the 200 active lines/field
• Repeat the above for the other field
• Build the de-interleaved frame
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Odd Field
Brightness and contrast adjusted for better projection
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Even Field
Brightness and contrast adjusted for better projection
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De-interleaved Frame
Brightness and contrast adjusted for better projection
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Algorithm contd…
• Comb Filtering– Luminance comb filter suppresses odd
harmonics of horizontal line frequency– Chrominance comb filter supresseseven
harmonics of horizontal line frequency
• Several Varients– 2 line– 3 line– 2 line Adaptive
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Comb Filter Action
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Unfiltered Image
Brightness and contrast adjusted for better projection
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Luminance Image
Brightness and contrast adjusted for better projection
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Luminance Samples’ FFT
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Chrominance Image
Brightness and contrast adjusted for better projection
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Chrominance Samples’ FFT
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Algorithm contd…
• I & Q components of color are DSBSC modulated– Amplitude roughly corresponds to Saturation– Phase roughly corresponds to Hue
• To decode color, we need a phase reference– Color burst provide a phase reference– It is 8-11 cycles of unmodulated carrier on back porch– Used as frequency and amplitude reference
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Color Burst
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Phase Information
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Color Image
Brightness and contrast adjusted for better projection
Color information not disturbed
Color Decoding Results based on preliminary work
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Conclusions and Future Directions
• Monochrome and preliminary color decoding results were presented
• Performance improvements possible by implementing a carrier tracking module
• Algorithms have to be tested and fine tuned with SMPTE bar
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Special Thanks to Eric Blossom and the entire GNU Radio community for their support and guidance
Thank You
References in the Thesis
Available at www.geocities.com/pmd_iitgw/thesis_prateek_iitg.pdf