Hi there everyone,

In another forum/site I was asking how could it be possible that digital audio could suffer from symptoms like non-constant playback speed (once was wow&flutter on old HIFI stereo).

I had this feeling when listening music from my phone connected to the Car Stereo.

While I thought the defect was due to poor hardware playing back music (phone + car kit), I was told that compressed digital audio can suffer from jitter/pitch issues (correct me if wrong), due to compression itself.

Basically sound wows, perceivable particularly on some instruments, like violin.

The offending music track has been downloaded from iTunes and has a 44Khz/256kbps/AAC format.

Could someone clarify the following:

- Are the speculations correct?
- Is there some software that can analyze a track and look for jitter (without having the original track), maybe tracking BPMs and verify if constant?

You can find out yourself by performing an ABX test between playback of wav and a compressed aac.

I don't have the original track (downloaded from iTunes)

Well I have another track that sounds "jittery", ripped off a CD. I could test that one. What tools do I need for the ABX test?

Then try with an original track? I suppose you "hear" this with all compressed music?

There's no jitter in audio files, regardless compressed or not.

To do an ABX with your phone, have someone to create a playlist with random entrys of the compressed and non-compressed song and a results-list that you can't see. Then play each track and note which one you believe it is. at any point you may request to hear A or B.

But to be honest: It's all in your head IMHO.

Isn't there an artefact called warbling? Could that be what the OP is referring to?

When I first read this on the other forum, my first thoughts were:
a) The phone was busy with other tasks and not paying attention to audio business.
b) The interface between phone and receiver was flacky and maybe playing the last data until it got fresh info.
c) The phone was dropping packets and doing some interpolation rather than muting.

- The phone connects via A2DP Bluetooth to the car kit (CK-300) wireless unit HF-21W that is plugged into the car stereo input.

- Audio "wows" can be heard randomly, not very often, I should say when a continuous sound such as a violin, a long whistle are played; it's not a proper "wow" (up & down) but seems rather "slightly slowing"....

Maybe I should try to play the same tracks on a "professional" system..... what system? Is my PC considered reliable for such a test, maybe using earphones?

Maybe I could send someone the tracks..... if only I wouldn't fall into copyright infringement....

Wikipedia:


Nokia Europe:


Maybe your phone sends AAC files to HF-21W via Bluetooth, and it decodes them.
But maybe the phone transcodes them to SBC or MP3. It's not clear to me.

Try to play these files without any wireless transmission.

Do you believe a PC is reliable?

I have ripped again the track with EAC and tried to spot differences on my PC.... both the files seem fine.

Does someone would enjoy testing? I can upload the two files somewhere....

P.S. - Still more convinced: the problem lies into the player

Lossy coding can cause loss of pitch precision. Its not usually an issue for the higher bitrate material you'd download from iTunes.

There are psycho acoustic issues involved in the perception of pitch. For instance, many sensitive listeners hear pitch differently through headphones than through speakers. I wouldn't be surprised if lossy coding affected pitch perception for some listeners. Are you the only one hearing these pitch problems or does someone riding in the car with you hear them too?

Or there could be something wrong with your player. Change in pitch is not a failure mode I've personally seen.

The phone could indeed be the problem. If it has an underpowered CPU that is running partly at 100% utilization and the output sample rate is generated in software, this could lead to heavy jitter with relation to the complexity of decoded AAC frames.

You will not find this distortion inside the iTunes track, when you play it back on a PC.

Note, that this can only happen, if a CPU is so slow, that it cannot decode an AAC stream in realtime without choking. For 99% of AAC capable devices this should not be the case and they output perfectly jitter free PCM streams. But maybe your device has either a too small CPU for AAC decoding or it is using an inefficient decoder or it is just a firmware bug.

In my (albeit limited) experience many A2DP implementations will not support anything beyond SBC and MP3. It is very likely that the phone will transcode to MP3 or even SBC on transmission, and not necessarily at a high bitrate. I think this is the most likely cause of the artifacts. Try connecting a good pair of headphones to your phone and see if the problem dissapears - if it does, it's an artifact of the Bluetooth transmission.

Nice to hear from you all.

It's only me having the perception of music slowing down.

I have developed this "sensitivity" from the days when (young) I've enjoyed playing music as a DJ: at that time you had to modify down/up very slowly speed play to match BPMs; even if done correctly you could always perceive slight speed variations. To explain better, this speed variation occurs not in a fraction of a second but in a span of 2/3 seconds.

That same occurs on the phone music player, sometimes.

On the phone forum, I have been told that if the phone CPU is under stress, the problem can arise. So yes, I really believe the problem lies in the phone. And yes, I believe the playback is done by software, I don't think there's any dedicated hardware doing the job.

BTW last night, I have played the offending track on my PC, using earphones; no problems with both the music formats: uncompressed WAV and AAC.

In the CPU stress hypothesis, audio samples or entire blocks would be skipped. You might be able to hear short dropouts or discontinuities. You might hear these as clicks of glitches. Using a pure tone or solo piano as source material would make them stand out.

That's not correct. When you have two separate processes or hardware units, one that pulls from a buffer at a specific sample rate and one that feeds the buffer with decoded samples, then there could be dropouts. If instead the output clock is generated by the same CPU, that does the decoding, jitter is certainly possible. Engineering a mobile phone involves a lot of trade-offs regarding costs, energy consumption, HF issues, and so on. Ruling out that those design choices could cause audible jitter, but instead could only manifest as dropouts, is just unsubstantiated.

Long shot, assuming these effects are for real: can periodic "device noise" -- when the CPU is working hard, it is usually receiving a signal of some kind -- make the sound slightly brighter/sharper and then darker/flatter? Like, when you scroll on your computer, the video card makes your headphones hiss. I guess some will interpret a sharper/flatter period as higher/lower speed?