This is my first post so forgive me if there is an existing topic on this subject. I saw one that touched on this lightly but it wasn’t enough to completely ease my mind.
I have purchased quite a number of mp3s from both Amazon and Google the past month when I noticed distortion in one of the files I purchased. Most of the files I have purchased I have not listened to yet because I naively assumed they would be quality files.
When I loaded the suspect file into Audacity to my shock and horror I saw there was much clipping. After checking a handful of files I have found only one file that was created within an acceptable audio range. How do they get away with this? Even when I convert my own CD purchases into MP3 I know enough to convert them under the clipping point.
Can anybody explain to me what is going on? Some of my files show more red than blue! It is a heart sinking feeling. Has anybody else ever corresponded with Amazon or Google or iTunes asking them to explain why their files are of such poor quality?
I have also noticed that all of the files I have purchased from Amazon that are listed with a 320kbps bit rate never actually play above a bit rate of 233kbps. The ones rated at 256 do play back at that rate. All of the ones from Google that are rated at 320kbps do play back at that rate even though many of them show clipping.
I feel like I have been had. Am I looking at this the wrong way? What am I missing?
Any help would be greatly appreciated.
Sincerely,
mpfried
It’s not as dreadful as you think. Start with a highly compressed and processed and legal music file. The peaks of the waves are just below or maybe exactly at 0dB. Now create an MP3 music file. MP3 is not an exact copy of the original work. MP3 plays very sophisticated games with the quality and content of the work – but in the process can actually cause the music to increase very slightly in volume. Since the music started out already at zero, that’s overload, although you’re not likely to hear it – or even see it on the blue waves (if you turn the red off).
This is a discussion we have about the behavior of the Audacity clipping indicators. They fire at zero and zero is not clipping. Exceeding zero is clipping.
You can also use Analyze > Find Clipping and that may give you slightly different numbers.
I remember I bought a performance that showed up as almost a solid block of red overload in Audacity. I was not, however, able to find any actual significant distortion.
Koz
Anything over 256 is pretty good and you’re unlikely to notice any serious compression damage. The minimum for stereo is 64 using a good compressor on original work, so 256 or anything above that is terrific. Audacity default is 128. I believe the highest pre-packaged compression bit rate on a Mac is iTunes Stereo 256. By the time you get into the 300s, only young girls are likely to hear the difference, if then.
Koz
Please also note that you only get to MP3 once, to coin a verb. If you try to edit a song that was compressed in 64, you will need to export it at 320 or higher to avoid any further compression damage. All the compressions on one song add up – and not in a good way.
Koz
…when I noticed distortion in one of the files I purchased.
Most-likely NOT the result of MP3 compression. It could be [u]The Loudness War[/u] and in that case, the same distortion will appear on the uncompressed CD. 
When you see “red” in Audacity, that doesn’t actually indicate clipping (squared-off wave peaks). It indicates the potential for clipping. Audacity can go over 0dB “internally” without clipping, and MP3 can also go over 0dB without clipping. If you decode/play the MP3 and send it directly to your soundcard/DAC without any digital volume control, or if you convert the file to WAV, then it will clip.
However, I’ve NEVER heard of a case where this slight-clipping was audible. That is, if you hear an MP3 artifact (or a difference between the uncompressed orignal and the MP3), encoding at a reduced level to prevent clipping will not improve the quality of the compression.
I’m not sure if there’s a relationship between the bitrate and the MP3 peaks… I wouldn’t be surprised if higher bitrate encoding actually increased the peaks, because there is more 'information" in the higher bitrate file.
And if you see a LOT of red, it’s a good indication that the music was highly compressed and limited, and perhaps clipped, before being MP3 encoded. So again, the CD probably sounds just as bad…
Thank you both kozikowski and DVDdoug for your generous feedback. Yea, I am not really that concerned about bit rate playback as long as it stays above 192kbps. I just don’t like the file being listed at a certain bit rate and then see that’s actually not the case. That just plain seems deceptive to me. I told Amazon I was perfectly happy with 256 because that is what the actual playback rate is. The ones listed at 320 actually play back lower than 256. The customer should be made aware of that up front.
I am off to read the The Loudness War before I post any more concerns about the clipping issue. All I know is that a co-worker of mine just happened to have purchased the exact same album from iTunes as one I purchased from Amazon. She was kind enough to bring her laptop to work and let me analyze them using Audacity and Spek. The very first one I loaded into Audacity from hers was one of the ones that looked like its throat was cut from my Amazon one. The results even made me sicker. Hers was a beautiful blue from one end to the other and the playback rate was exactly as listed. I also ran the Analyze->Find Clipping and the results were perfect. I sent my results to Amazon and they asked me to be very patient while their tech gurus look into it.
I guess my problem is that I have been recording vinyl to tape and both vinyl and tape to digital since I was small. I learned by trial and error the optimal input volume to record at. It is a balancing act between a level low enough to avoid clipping above 0db and yet high enough to mask line level noise and any background white noise you get when recording through a mic.
I have spent countless hours doing this and using everything from the finest turntable cartridges and styluses to MAudio to Nero Wave Editor, Goldwave, and Wave Repair for click removal. Unfortunately for my psyche, this has turned me into a perfectionist. Regardless of whether there is actual distortion or not any signs of clipping above 0db is absolutely unacceptable to me. I would think most of the artists themselves would not want their hard work reproduced at a substandard quality.
Anyway, I will read about The Loudness War (sounds like an interesting read for a nerd like me) and I will post any more concerns that come to mind.
Again, thank you very much for your help.
mpfried