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We record a lot of dj sets. To do this we use this setup:
- We use the rec out from the dj mixer.
- From the rec out, we use a short cable to 2 passive DI boxes
- From the DI boxes, 20 meter XLR runs to our Zoom R16
- In the Zoom R16 we mix in a mic pointed at the crowd.
- The outputs of the R16 are going to a Focusrite 2i2 that is connected to the pc via usb
- in vmix we select the 2i2 as audio source
- In Vmix we use the mpeg TS codec to record the show
When i analyse the output file, the bitrate of the audio is 384kbps, 16 bit, which is supposed to be very high quality
When i export the audio of the recorded file to pcm wav though, and import it into adobe audition, i can see that the frequencies above 15 kHz have been cut off. This is really bad, because it corresponds to MP3 128kbps audio quality.
Sometimes we do backup recordings on the zoom r16, those backup files are pcm .wav and look allright in audition.
Could it be that the internal mpeg encoder makes a large file, but somewhere along the way the audio is compressed in the software?
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Bump
It's really important for us to have a clue about this problem, since quality audio is mega important for us. Anyone?
If not, anyone who knows how we can record on the same computer with a good capture software via virtual cam?
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Ittaidv wrote:...because it corresponds to MP3 128kbps audio quality... No, because you can also encode to a 320kbps MP3 with a high cut set to 15kHz. A high cut filter has nothing to do with the bitrate. MPEG-2-TS encodes your audio to MP2 (inside the video file) and maybe there's an FFMPEG encode option set for a high cut. But for high quality audio you wouldn't use MP2, correct? So you have to try and use another setup for your recording. For example using AVI with the Magic YUV lossless codec will encode your audio to uncompressed PCM Wave. The disadvantage is a much bigger video file. I did not try all the other codecs so it's up to you.....
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ovinas wrote:Ittaidv wrote:...because it corresponds to MP3 128kbps audio quality... No, because you can also encode to a 320kbps MP3 with a high cut set to 15kHz. A high cut filter has nothing to do with the bitrate. MPEG-2-TS encodes your audio to MP2 (inside the video file) and maybe there's an FFMPEG encode option set for a high cut. But for high quality audio you wouldn't use MP2, correct? So you have to try and use another setup for your recording. For example using AVI with the Magic YUV lossless codec will encode your audio to uncompressed PCM Wave. The disadvantage is a much bigger video file. I did not try all the other codecs so it's up to you..... If I would be using a highcut filter somewhere in my setup I would understand. But I don't use any effect, except for compression. Mpeg audio at 384kbps should be almost lossless. If I can extract a proper 320 file from the recording I'm already happy, no need to record in PCM Wav. I will do trials with other codecs, to check if it is a codec problem or if the mistake is happening somewhere else in our setup. It looks to me like somewhere along the route, the audio is compressed to 128kbps before it's recorded in Mpeg. We sometimes make backup recordings on our Zoom R16, and those are proper WAV files, with all frequencies present. Since there is no audio degradation happening between the R16 and Vmix, I don't understand where the audio is compressed or filtered along the route. Btw, recording a 12 hour night in uncompressed AVI is not an option for us :)
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Once again, the bitrate has absolutely nothing to do with a high cut filter. When encoding a wave file to MP3 using LAME for example you can set an option for a high cut filter independent from the bitrate. You can do it with 128kbps, with 384kbps or any other bitrate.
Using MPEG-2 recording in vMix will trigger FFMPEG with many options/settings to do the encoding in the background. One option could be that high cut. Or it is on by default and the high cut has to be disabled. Nothing a user could change because it's under the hood of vMix.
Magic YUV lossless is not uncompressed (for the video part).
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No, it doesn't! Reading your links you will see that they are using presets. Quote:The default presets for mp3 encoders have a standard frequency cutoff specified for each bitrate...
Here are the different default cutoffs for mp3 at different bitrates:
320kbps - 20.5 kHz
256kbps - 20 kHz
192kbps - 19.5 kHz
128kbps - 16 kHz So if you use a default preset the high cut will be set. What happens if you don't use a default preset and set your own options? And if you read this very carefully Quote:but these settings can be changed manually. What this means is that just because the spectrals for an audio file show a filter cutoff at a certain frequency does not mean that it has been encoded at a certain bitrate. it's exactly what I'm trying to explain...
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Yeah ok, you are right about that. This brings me already a bit closer to findig a solution, thanks for that.
But that doesn't change that this cutoff at 15 khz can't be switched off in Vmix, since there are no audio options at all for mpeg.
Using an uncompressed codec is also impossible for us, since investing in SSD's to contain 12 hour non-stop footage is a bit too much, and loading this into premiere for editing would require a monster pc and a lot of time.
Using mp4 is also no option, because of the 4gb file limit, and it takes to much processing power. WMV is an option i could try, since it has audio options, but I don't really like this codec for editing, and suppose it takes a lot of cpu as well?
Any other suggestions, or a way to 'hack' into the configuration file for mpeg streaming?
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Hi,
The MPEG-2 recording uses the broadcast standard MPEG-1 Layer II audio encoding format.
(This is different to MP3, which is why the higher bitrate is needed for the same quality)
As for the audio cutoff, if you do a test of a 20khz audio tone then run the resulting TS recording
through a spectrum analyzer you will see the signal appears as expected above 15khz.
What you will also see is the noise floor above 15khz gets cut as well, because the audio compression
will eliminate certain frequencies that are too low to be audible and would be wasted bits to encode.
Hope this helps.
Regards,
Martin
vMix
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Rank: Administration
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Joined: 1/13/2010(UTC) Posts: 5,208 Location: Gold Coast, Australia Was thanked: 4289 time(s) in 1520 post(s)
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Also, for raw audio recoding, try the AVI format with the built in MJPEG Encoder DMO codec.
(Included with the latest vMix 14 release)
This will create larger file sizes, but still manageable, under 20GB per hour.
Regards,
Martin
vMix
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