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Topic: MPC Treshold (Read 5577 times) previous topic - next topic
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MPC Treshold

I am told that with MP3 there is a threshold, above 128 the file gets 25% larger and the sound gets 1% clearer.
- So does MPC have one?
- If so what is it?

MPC Treshold

Reply #1 –
[deleted]

MPC Treshold

Reply #2 –
[deleted]

MPC Treshold

Reply #3 –
How do you relate something like subjective quality to percentages?

--
GCP

MPC Treshold

Reply #4 –
Garf,

I found this at an R3mix thread : I am fighting the trend of bloated MP3. A lot of these settings are getting up to about 240 kbps. That's only 5:1 compression. Honestly there is a leveling off point where the audio quality gain doesn't match the file size. Fraunhofer has claimed this threshold to be in the area of 112-128. I agree as well. Beyond this the file gets 25% bigger and the audio gets 1% clearer at each step.

so is this true?

MPC Treshold

Reply #5 –
Quote
Originally posted by westgroveg
Garf,

I found this at an R3mix thread : I am fighting the trend of bloated MP3. A lot of these settings are getting up to about 240 kbps. That's only 5:1 compression. Honestly there is a leveling off point where the audio quality gain doesn't match the file size. Fraunhofer has claimed this threshold to be in the area of 112-128. I agree as well. Beyond this the file gets 25% bigger and the audio gets 1% clearer at each step.

so is this true?


I repeat what I said earlier, namely how do you relate subjective quality to percentages?

Asking if its true is like asking if 5 is more true than green.

--
GCP

MPC Treshold

Reply #6 –
First thing, the space percentage: a quick computation shows that, as an example 256 is not 25% more than 224, even if it's the next step.

Second thing, the 1% clearer sound: this is a much true as the fact that FhG is claiming that 128kbps is cd quality.

MPC Treshold

Reply #7 –
Ok, in MP3 there certainly exists a so-called 'sweet spot' which is somewhere around 128 kbits/s. For AAC, the 'sweet spot' is around 96 kbits/s

Performance starts to fall suddenly when you get below the 'sweet-spot'. This value is dependent on huffman codebook efficiency and some other things (coding tools, filterbank...)

But that doesn't mean that higher bitrate is not necessary - there are many cases where PE (perceptual entropy) is much higher than 1.4 bits per sample (11:1 compression) and in that case 128 kbits/s MP3 or AAC is simply not enough.

For MP3, another problem arise - filterbank inefficiency when compared to new advanced filterbanks (AAC, Ogg, ...), big short block length and the last scalefactor problem  - all of these require higher bitrate than bitrate estimated by PE. Because of that a bitrate of 192-256 kbits/s is required for MP3 to be transparent in perceptual point of view.

MPC Treshold

Reply #8 –
Thanks for clearing that up Ivan  I now understand a bit better but does MPC have a 'sweet spot' ?

MPC Treshold

Reply #9 –
It has, of course - this spot is lower than MP2 because MPC has huffman coding step - but I am sure that Andree can answer this question much better. He has probably done some experiments.

Also, in this current implementation, MPC bitrate control is fully driven by psychoacoustic model - so there is no CBR and way to test quality degradation based on fixed bitrate conditions.

MPC Treshold

Reply #10 –
thanx for the info  Ivan