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Topic: File size of ITune AAC clips (Read 6845 times) previous topic - next topic
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File size of ITune AAC clips

When I encode castanets.wav with ITune AAC at 128kbps stereo, the AAC file size is about 160kbytes? It should be at about 103kbytes. ITune seemed to have included a huge header in front of the audio bitstream..

I was wondering, if a conformance decoder will be able to decode a AAC file which consists only the ADTS or ADIF header? In Mp3, the headers that exists is something like the AAC ADTS header.. and without the transport stream container format..

File size of ITune AAC clips

Reply #1 –
Hmm, you're right 

What iTunes does is that it reserves some (ok, a lot of) space for tag editing. This can be considered quite smart because tags can be edited without rewriting the whole file structure every time and the "moov" box can always (unless you add more metadata then reserved space) stay at the front of the file.

So this extra data has nothing to do with the actual AAC data in the file, it's part of the MP4 file structure. On small files this reserved space is just noticed a bit more than in longer files.

Menno

File size of ITune AAC clips

Reply #2 –
Quote
What iTunes does is that it reserves some (ok, a lot of) space for tag editing.

32kB isn't it? I believe they do the same thing with MP3's, and pad them to make tag editing quicker.

Speaking of overhead, do any of you know how MOV/MP4 compare to the other formats in that area?

File size of ITune AAC clips

Reply #3 –
Quote
Speaking of overhead, do any of you know how MOV/MP4 compare to the other formats in that area?

MP4 is quite compact. AAC in MP4 is a lot smaller then AAC with ADTS headers for example. I don't have any exact numbers or comparisons with other container formats.

Menno

File size of ITune AAC clips

Reply #4 –
Encode a one second sample : QT encodings are 50 kb higher than other formats at the same bitrate.

You can lower the space with foobar2000 (optimise MP4 layout): that's what I did for my listening test. I therefore obtained a real 128 kbps filesize. You can even extract the aac stream, and then convert to mp4 to obtain the same result (but you will lose some extra info, as encoder name & version - it's longer too).
Try wih foobar2000 "optimize mp4 layout" (context menu) first.

File size of ITune AAC clips

Reply #5 –
Quote
Encode a one second sample : QT encodings are 50 kb higher than other formats at the same bitrate.

You can lower the space with foobar2000 (optimise MP4 layout): that's what I did for my listening test. I therefore obtained a real 128 kbps filesize. You can even extract the aac stream, and then convert to mp4 to obtain the same result (but you will lose some extra info, as encoder name & version - it's longer too).
Try wih foobar2000 "optimize mp4 layout" (context menu) first.

The bitrate info in foobar gives the correct average AAC bitrate. No matter how much overhead the MP4 file format takes.

Menno

File size of ITune AAC clips

Reply #6 –
Maybe, I don't know... I talked about file size. All encodings I did with iTunes CBR 128 had the same size, something like 520 kb. It was ~50 kb excessive, compared to others 128 encodings. After the foobar optimisation, size dropped to 476 kb, which correspond to the theorical size for 128 bkps bitrate (and then, foobar2000 rated the bitrate to 124 kbps, which is wrong).

File size of ITune AAC clips

Reply #7 –
Quote
Maybe, I don't know... I talked about file size. All encodings I did with iTunes CBR 128 had the same size, something like 520 kb. It was ~50 kb excessive, compared to others 128 encodings. After the foobar optimisation, size dropped to 476 kb, which correspond to the theorical size for 128 bkps bitrate (and then, foobar2000 rated the bitrate to 124 kbps, which is wrong).

So the mp4 overhead was 128-124 = 4 kbps for that file. The bigger the file the smaller this becomes. The reported average bitrate in foobar is the bitrate of the RAW AAC data.

Menno

File size of ITune AAC clips

Reply #8 –
What is overhead? By comparing size and bitrate estimation in fobar for the same sample, I obtained :

Code: [Select]
AAC iTunes    476 kb   124 kbps
AAC Faac      474 kb   124 kbps
AAC Nero CBR  479 kb   124 kbps
WMA ABR 128   476 kb   128 kbps
MP3 Lame 128  476 kb   128 kbps
MPC --thumb   472 kb   129 kbps [unable to reach 476 kb]
OGG [big jump between 2.99 and 3.00]

File size of ITune AAC clips

Reply #9 –
With overhead I mean the space taken by the container format. MP4 gives the bitrate of the actual audio data and does not take into account the data that the container itself takes. Otherwise the bitrate would be a little bit off if you added a 2 MBit video track 

Menno

File size of ITune AAC clips

Reply #10 –
OK. Thank you for this clear explanation
P.S. It means that, for exemple, iTunes and Nero AAC are not really 128 kbps, but 124 kbps for audio stream. In other words, people complaining about small bitrate deviation in ABX test have here another argument for bothering testers

File size of ITune AAC clips

Reply #11 –
When I built the bitrate tables for my tests, I always used the bitrate reported by Foobar for MP4. I didn't do filesize/time.

File size of ITune AAC clips

Reply #12 –
And what about Faad informations in CEP/Audition? They are very precise (124207 bps for exemple), but are they reliable? And what does "optimize mp4 layout" exactly do? Is it safe to use it? Are problems possible with portable players as iPod, or for video mux? Thanks

File size of ITune AAC clips

Reply #13 –
If "optimize MP4 layout" is what I think it is - it shouldn't affect compatibility at all - it just reorganises the file and optimizes it for less seeking.

File size of ITune AAC clips

Reply #14 –
Quote
If "optimize MP4 layout" is what I think it is - it shouldn't affect compatibility at all - it just reorganises the file and optimizes it for less seeking.

When I performed my test, I gained a consequent amount of space with iTunes encodings (based on 30 sec. samples), but unsignificant one with Nero AAC or FAAC encodings.

I didn't notice anything for seeking, but as an mpc user, I'm used to have low seeking, an mp4 is really fast in comparison

Anyway, nice work for your promising new AAC encoder. It seems that finding difficult samples for it will be more difficult than ever (and performing ABX tests too).