This is the spectrogram display of a 16-bit, 44100 Hz sampling rate input:
In this case, downsampling this file into 22050 Hz sampling rate would not suffer from noticable quality loss.
Consider this one:
Nyquist–Shannon sampling theorem suggests that we need at least a sampling rate of 44100 Hz. Otherwise, it will not be possible to cover the whole frequency range of the input.
So, the required sampling rate depends on the input.
Quote:
Originally Posted by fenomeno83
use aac he vers 1 to encode film with bitrate between 48 and 80 kb/s (64 is suggested) using 32000 sampling(44100 isn't necessary ?),stereo
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Assume that we have a stereo input similar to the one in the second figure which requires a sampling rate of 44100. Our target bitrate is 64 kbit/s.
The starting point should be the following:
At this bitrate level, can the aac encoder produce good results for 16-bit, stereo, 22050 Hz input? The reference table in my previous post suggests that the bitrate range of 45-70 is likely to produce very good results for such input.
So, the underlying coder will be able to successfully transmit the lower part of the spectrum. On the other hand, the higher frequencies will be reconstructed by the SBR decoder using the information included in the bitstream.
In conclusion, at 64 kbit/s it makes sense to stay at 44100 Hz and rely on SBR.
Quote:
Originally Posted by fenomeno83
use aac he vers 2 only for streaming application with bitrate between 16 and 40 kb/s using 22050 sampling..but in this case is better use mono
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Again referring to the table in my previous post, a bitrate level in the range 45-70 is likely to produce very good results for 16-bit, stereo, 22050 Hz input. If our target bitrate is only slightly below this (40 kbit/s for example), parametric stereo is not really needed. AAC+SBR will be the optimal choice.
So, when do we need HE-AAC v2 (aac+sbr+ps)?
Assume that we have a 16-bit, 44100 Hz, stereo input. Our target bitrate is 32 kbit/s. At this bitrate level, can the aac encoder produce good results for 16-bit, stereo, 22050 Hz (half the original sampling rate) input? The table suggests that we need 45-70 kbit/s, so the answer is no. From this information, we might expect that HE-AAC v1 will not produce satisfactory results.
The next question is the following:
At bitrate levels slightly below (some datarate should be reserved for guidance sbr and ps info) our target bitrate (32 kbps), can the aac encoder produce good results for 16-bit, mono, 22050 Hz input?
The answer is yes. In this case, it makes sense to use HE-AAC v2.
p.s.
What I have written here are somewhat cheap generalizations relying on a brief reading. Actually, I don't have any background on this subject.
I suggest that anyone reading my post should consider this.