Frank Klemm's Technical Page
Time resolution of Analyse + Synthese
The subband encoders MP1, MP2 and MusePack/MPEGplus have the best time
MP3 has the worst time resolution.
(MPEG-2) AAC something
The time resolution of MP3 and AAC can be switched (you see the 2 graphs
for both, one solid, one dotted). You have one for tonal signals, one for
But note that for MP3 and AAC the time resolution is not constant and
jitters depending on the position of the sample
in the transformed block. The diagram shows the average. If the coder is not well tuned this
non constant time resolution generates quick changing artefacts.
A critical testcase is Short_Block_Test_2.wav.bz2.
MPEG-4 AAC Main Profile provides an additional feature called TNS: Temporal Noise Shaping.
This provides additional time resolution.
A logarithmic plot over 110 dB of dynamics:
More interesting is the area from -25 . . . 0 dB:
Why have MP3 and AAC less time resolution?
- You can see that MP1, MP2 and MPEGplus have less problems with pre-echos.
- MP3 has much problems with pre-echos
- MPEG-2 AAC something between
- MPEG-4 AAC uses TNS and can be better than MP1, MP2 and MPEGplus
- They have a higher frequency resolution and a higher frequency
resolution means a lower time resolution because these two
properties are coupled.
- This higher frequency resolution has advantages if you encode
highly tonal signals, but makes lots of trouble on transient signals
- Therefore MP3 and AAC uses blocks switching between two different
- To select the right blocks size is difficult to calculate, and
for MP3 even the short block is too long for percussive signals.
- To my mind only MPEG-4 AAC is capable to eliminates all
disadvantages of the additional frequency resolution.
The result is transparent coding at data rates around 120...130 kbps
(instead of 170...180 kbps as MPEGplus).
- high quality MPEG-4 AAC Encoder is much much more difficult
to program and to tune than a MPEGplus encoder
- Is by a factor of 10...30 slower
- also the decoder needs much more powerful hardware
(most hardware can only play MPEG-2 AAC Low Complexity Profile)
High quality encodings with 32 kHz.
Time resolution for 32 kHz PCM source is 38% less than at 44.1 kHz and 50% less than 48 kHz.
If you have high quality PCM input with 32 kHz (DAT longplay, DSR) and you want to encode it as
MP3 or MPEG 2-AAC it is necessary to resample it to 44.1 kHz (441:320) or 48 kHz (3:2), depending on the target.
Last modified: 2001-11-28 Visitors: