Trying to redo this thread since JMAC deleted all his posts...
COPIED from the WHAT.CD Wiki Tutorials:
What is a transcode?
Wikipedia says that "Transcoding is the direct digital-to-digital conversion from one (usually lossy) codec to another." A transcode is any conversion of format.
Why is lossy transcoding bad?
Whenever you encode a file to a lossy format (such as mp3, m4a(AAC), ogg, or mpc) information is permanently lost. It doesn't matter what you do, it's impossible to get this information back without making a new rip from the original lossless source. If you re-encode it to a different format or bitrate, you are reducing the quality. This applies to any lossy to lossy conversion, so even if you convert from 320kbps to 192kbps, the final file will still sound worse than if you had just ripped to 192kbps in the first place.
It's also important to remember to verify that lossless rips actually came from an original source. People that download lossless expect it to be identical to the original. There's no point in people downloading a bigger file just to get another lossy rip.
So how do I verify that my music isn't a transcode?
The simplest way is to rip and encode it from the original source yourself. That way, you know that there has been only one lossy step (or that the rip is truly lossless, if you decided to do a lossless rip).
You should also check it by using a wave editor (such as Adobe Audition) to look at the spectral frequency display.
What is the difference between FhG and LAME?
Most lossy encoders use a low-pass filter when encoding. The filter is set to cut frequencies above a certain point and leave those below. The reason they're doing it is, that high frequencies are more difficult to encode and hearing is less sensitive in higher frequencies. MP3 encoders at 128kbps will typically use a LPF at 16kHz. As you raise the bitrate, the frequency threshold raises. At 192kbps the LPF is usually set at 18kHz or higher.
How can I view the Spectral Analysis of songs using Adobe Audition?
To view the spectral analysis of audio files in Adobe Audition, first ensure you are in Edit Waveform View by pressing the number 8 on your keyboard. Then, go to File > Open and select the file you wish to test. Adobe Audition will open the audio file in the "Waveform View" by default each time, so you'll need to choose View > Spectral View or press F9 to switch to Spectral View.
Common Bitrate Comparisons
The following section contains a list of common bitrates and their audio spectrum. The LAME were all done using dbpoweramp from a flac source, and they are all encoded using LAME version 3.97. The FhG were all done using Adobe Audition 1.5 FhG. The shape of the screen-shots differ due to different screen resolutions.
V2 (preset standard, aka aps):