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· Hall Of Fame
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Discussion Starter · #1 ·
With my HR20-100, I noticed that when changing channels between ch80 and ch82 it takes about 2s, but between 390 and 392 it's 3s.

I assume this is due to the difference in MPEG2 and MPEG4 decoding of the stream.

Just wondered if anyone else has noticed this?
 

· Banned
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294 Posts
Nope, never actually have noticed it. I think it takes no long to change channels regardless if its 2 or 3 seconds, but than again I am used to analog cable where it was practically no time at all.
 

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Discussion Starter · #4 ·
I only noticed it as I was watching the golf this past weekend on chs 390 and 392 (newly turned on MPEG4). I was checking PQ between ch 80 and ch 390 and 82 with 392. I was frustrated in that when changing the channel that it took so long. Then did the ch 80 to 82 timing and then 390 to 392 test.

Obviously, the unit needs to acquire the channel (select satellite and TP) and then decode it. I wonder if a software upgrade could help channel changing?
 

· Broadcast Engineer
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jdspencer said:
...I assume this is due to the difference in MPEG2 and MPEG4 decoding of the stream...
You are correct, grasshopper! Very astute observation.

MPEG uses a "group of pictures" where certain sorts of frames in the GOP (B and P frames) are dependent on other frames (I frames) to be sent before decoding. This is primarily what accounts for the latency, waiting for successive frames before decoding them into video frames.

When you tune to a channel, typically in the middle of the GOP, you have to wait for the GOP to end (and an I frame to be sent) before you get acquisition, because the P and B frames sent before that I frame have no reference and can't be decoded. During the acquisition delay, the video is muted to black.

The GOP for MPEG-2 (as used by ATSC and DVB HD) means an I-frame will come along every half-second or so. For MPEG-4, I frames can be as far as 200 frames apart (6 seconds or more) although they are typically sent by DTV more often to prevent having to wait that long for acquisition. Camera shot changes are also sometimes accompanied by an I frame to prevent pixellation. This is the very same reason FFWD is smoother with MPEG-2 than MPEG-4.
 

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TomCat said:
You are correct, grasshopper! Very astute observation.

MPEG uses a "group of pictures" where certain sorts of frames in the GOP (B and P frames) are dependent on other frames (I frames) to be sent before decoding. This is primarily what accounts for the latency, waiting for successive frames before decoding them into video frames.

When you tune to a channel, typically in the middle of the GOP, you have to wait for the GOP to end (and an I frame to be sent) before you get acquisition, because the P and B frames sent before that I frame have no reference and can't be decoded. During the acquisition delay, the video is muted to black.

The GOP for MPEG-2 (as used by ATSC and DVB HD) means an I-frame will come along every half-second or so. For MPEG-4, I frames can be as far as 200 frames apart (6 seconds or more) although they are typically sent by DTV more often to prevent having to wait that long for acquisition. Camera shot changes are also sometimes accompanied by an I frame to prevent pixellation. This is the very same reason FFWD is smoother with MPEG-2 than MPEG-4.
TomCat, I want to thank you for your excellent explanation and hope that you continue with such informative messages!
 
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