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Instant replay quality at different bit rates?
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jip  
#1 Posted : Sunday, April 12, 2015 12:25:51 PM(UTC)
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Hi all,
I'm testing the instant replay feature with 1080i50 material captured through HD-SDI with Blackmagic Decklink Duo. The image quality in the camera capture is good.
However, it seems that there is no clearly perceivable different whether the instant replay is recording at 25 Mbps or 100 Mbps. It looks as the recording is grainy and blocky even at 100 Mbps, although the source material should be uncompressed.

I would have expected that 100 Mbps bit rate would be practically indistinguishable from the original capture.

Have anyone else ran into this issue?

(I'm using version 15.0.0.74 x64)
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Kibim  
#2 Posted : Saturday, June 6, 2015 11:30:37 AM(UTC)
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jip wrote:
Hi all,
I'm testing the instant replay feature with 1080i50 material captured through HD-SDI with Blackmagic Decklink Duo. The image quality in the camera capture is good.
However, it seems that there is no clearly perceivable different whether the instant replay is recording at 25 Mbps or 100 Mbps. It looks as the recording is grainy and blocky even at 100 Mbps, although the source material should be uncompressed.

I would have expected that 100 Mbps bit rate would be practically indistinguishable from the original capture.

Have anyone else ran into this issue?

(I'm using version 15.0.0.74 x64)


Yes, i get the same issue. What i found is by creating a new replay session, ie : deleting all previous files, or changing recording directory, it then accepts the new bit rate.
25mbps is not really usable, unless you are only streaming a very low resolution, i find 50Mbps is acceptable quality for most applications.
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jip  
#3 Posted : Sunday, June 7, 2015 1:45:30 AM(UTC)
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Kibim wrote:

25mbps is not really usable, unless you are only streaming a very low resolution, i find 50Mbps is acceptable quality for most applications.


Thanks for the comment!

Could this requirement for a relatively high bitrate be due to the format of stored replay material. I mean, the encoding is rather unefficient compared to avchd or mpeg4 etc?
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thomas  
#4 Posted : Thursday, June 11, 2015 9:20:08 AM(UTC)
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I have just tested 50mbps and 100mbps with vectorscope and Trimaster monitors. 50mbps is (in my opinion) way to blocky and compressed. 50mbps is usually the default recording-quality for broadcast. But in vMix it is too compressed. I hope this get resolved in an update.

100 mbps is also a bit compressed, even though the original source is 50mbps 1080i50. I love vMix and the possibilities, but this should be prioritized, something weird is going on with the compression.

jip wrote:
Kibim wrote:

25mbps is not really usable, unless you are only streaming a very low resolution, i find 50Mbps is acceptable quality for most applications.


Thanks for the comment!

Could this requirement for a relatively high bitrate be due to the format of stored replay material. I mean, the encoding is rather unefficient compared to avchd or mpeg4 etc?
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admin  
#5 Posted : Thursday, June 11, 2015 1:06:15 PM(UTC)
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Hi,

To clarify how the bit rates work, for performance reasons they are recorded as MPEG-2 I-Frame compression.
The I-Frame is important as it allows random access to each frame in the recording.
(This is what is also used on other professional instant replay systems)

The downside is it requires much higher bitrates for the quality as standard MPEG-2.

So 100mbps is recommended for 1080 sources and even higher bit rates options will be available in the next version of vMix.

Regards,

Martin
vMix
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jip on 6/12/2015(UTC)
jip  
#6 Posted : Friday, June 12, 2015 3:05:53 PM(UTC)
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admin wrote:
Hi,

To clarify how the bit rates work, for performance reasons they are recorded as MPEG-2 I-Frame compression.
The I-Frame is important as it allows random access to each frame in the recording.
(This is what is also used on other professional instant replay systems)

The downside is it requires much higher bitrates for the quality as standard MPEG-2.

So 100mbps is recommended for 1080 sources and even higher bit rates options will be available in the next version of vMix.

Regards,

Martin
vMix



Thank you Martin! This answers my question perfectly.
I would suggest an addition about this information about applicable bitrates to the respective section in the help documentation.
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