m4a to wma - Why such variation in bitrates?

[Est]

Hi there;
I am trying to use DB poweramp music converter to convert .m4a songs that I have from itunes into wma so that I can use them with windows media player (although I hve the codec so WMP can play .m4a, it doesn't have as much tag support as for .wma's).

Now, I have tested converting using the same setting on a couple of different .m4a songs, (all of them have the same 128kbps bitrate), but I get different results for each song!

For example:
Song 1 (AFI) - 128kbps .m4a
Song 2 (Al Green) - 128 kbps .m4a

I convert both using wma 9.1, quality 90 and 75, at 44k sampling, stero VBR
Outcome:
Song 1 at quality 90: 214kbps (Far too big)
Song 2 at quality 90: 265kbps (Even bigger)
[COLOR=DimGray]Song 1 at quality 75: 131kbps (That's more like it)
Song 2 at quality 75: 159 (Acceptable, but still much higher than song 1)[/COLOR]

Then I tried both with wma 9.1 professional, with 2 channel 24bit VBR, at quality 75, 50 ans 25:
Song 1 at quality 75: 139kbps (Good)
Song 2 at quality 75: 170kbps (Way too big!)
[COLOR=DimGray]Song 1 at quality 50: 116kbps (Good)
Song 2 at quality 50: 144kbps (Acceptable, but still much higher than song 1)[/COLOR]
Song 1 at quality 25: 102kbps (Too low)
Song 2 at quality 25: 127kbps (Acceptable, but still much higher than song 1)


Why do I get such drastically different output bitrates from songs which have the same bitrate when they go in?

Is there anything I can do to get songs that, when converted, come out with similar bit rates (and therefore file sizes) as the originals?

Thanks, Est

[Spoon]

Re: m4a to wma - Why such variation in bitrates?

That is VBR for you, it could be slightly thrown off because you are tanscoding (ie going from one already compressed format to another), it is best to rip from cd again.

[Est]

Re: m4a to wma - Why such variation in bitrates?

Ripping from cd again would probably be too time consuming and expensive...

Is there really no way to set the bitrate at which the file should be converted to .wma at (so I get the same bitrate on all converted files), as opposed to the 'quality' as it is called?

Edit:
I could just use a constant bit rate..

Is there any difference in overall sound quality between CBR and VBR? Or does it just have to do with compression? Any other differences I should be aware of before using a CBR rather than a VBR?

[LtData]

Re: m4a to wma - Why such variation in bitrates?

VBR normally sounds better, as it uses a higher bitrate when needed but a lower bitrate when it is not needed, resulting in about the same average bitrate but higher quality for the most part.

[Est]

Re: m4a to wma - Why such variation in bitrates?

Trouble is, I'm not getting the same average bit rates; they're all over the place.

Would you recommend that I just use CBR?

[LtData]

Re: m4a to wma - Why such variation in bitrates?

What do you mean "all over the place"? Are you getting variation when using the same quality setting above like +-5? Do note that two encoding-runs to VBR will probably result in a bitrate that is a little different (like 190 vs. 185). Changing the quality changes the bitrate range it uses.

[xoas]

Re: m4a to wma - Why such variation in bitrates?

I believe that you (Est) are noting a phenomena that seems to be common among vbr encoders: different songs encoded to the same Quality level vbr setting may have drastically different low, high and average bitrates.

This is disorienting for those who are used to equating quality with bitrate. As LtData has been explained, the nature of vbr encoding is to encode at higher bitrates when needed and at lower bitrates when it is not. This means even though file a and file b have been encoded at the same cbr bitrate, they may have very different vbr bitrates if both are converted to vbr. What should be constant, however, is that both files should preserve the same level of quality.

One other thing that is difficult for some users to get used to is that when ripping or encoding to vbr, it is more difficult than with cbr to predict what size the converted file will be.

In this particular case, it looks like you (Est) have found that the Q level of 50 for wma 9.1 professional seems about right for encoding in terms of quality. I might recommend checking out wma 9.1, stereo, 44.1 kHz at Q level of 50. This might be better for your needs. However, you may be more comfortable encoding to cbr.

Hope this helps.
Best wishes,
Bill

[gameplaya15143]

Re: m4a to wma - Why such variation in bitrates?

just a suggestion: if those m4a files are not encrypted, just use winamp with the mp4/aac input plugin from rarewares.org, full tag support (and not to mention a better player than wmp)

wma vbr uses a different quantitizer than vbr mp3, than vbr vorbis, than vbr aac, etc. etc. it shouldn't be any suprise that the bitrates are very different. if you are looking for ~128kbps, then encode in cbr