How to determine the true bit rate of any audio file

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We recently explained why ripping 320kbps MP3 from Youtube is a bad idea, but today we are going to talk about how you can check the true flow of any audio file you download – legally, we hope. It is in fact very useful, because many legal Music download sites may claim to offer CD-quality file downloads, until you actually inspect the bitrate.

For this task we can use Spek Spectrum Analyzer, which will tell you the cutoff frequency of any audio file as you feed it. Audio bit rate is intrinsically related to frequency, but we’ll explain that later.

Conditions:

  1. First install Spek on your PC and launch it.
  2. Now find an audio file (MP3, WAV, FLAC, AAC, whatever) for which you want to find the true bitrate.
  3. Drag and drop it into Spek – which will now display the frequency spectrum of the file.

The general rule is as follows:

  • Cut off at 11 kHz = Bitrate of 64 kbps.
  • Cut off at 16 kHz = Bitrate of 128 kbps.
  • Cut off at 19 kHz = Data rate of 192 kbps.
  • Cutoff at 20 kHz = Bitrate of 320 kbps.
  • Cut off at 22 kHz = Bit rate of 500 kbps.
  • No Cut = Bitrate greater than 1000kbps, you usually only see it with true lossless formats (WAV, FLAC).

So now let’s do some examples. We will extract the audio from a Youtube video that claims to be “lossless” audio, because the video was uploaded in MKV + FLAC – however, Youtube compresses its audio.

So let’s see the difference between extracting audio from Youtube to 128, 192, 320 AAC, 320 MP3 and FLAC:

As you can see, the cutoff frequency is exactly the same for all files, although some extra transcoding noise has been added to the “oversampled” rips, which will be pretty much static white noise in your headphones.

But we already knew that Youtube compresses its audio regardless of what format you download in, so let’s try a legal audio site. For this we will use a (legally purchased) iTunes track, which looks like a 256kbps M4A, with AAC encoding, which will actually give us a variable bitrate:

So even though this is a 256kbps M4A, it has plateau peaks in the 20-22kHz range, indicating a high quality export, most likely from an original of studio. The reason the frequency spectrum is not constant in all areas is the variable bit rate, so we need to be careful about the location of the plateau peaks. So here, we get what we pay for, iTunes is not fooling us.

Either way, you should always use this method of spectrum analysis when you want to know if you are really getting your money’s worth on a website that claims to sell “high quality” or “lossless” music.

Good listening !

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