how to get amplitude of mp3 audio file?

I need to know the amplitude value of a mp3 file to calculate PSNR. Is there any function in matlab that can show the amplitude value of a mp3 file? May someone help me to answer my question please? thank you.

Answer 1

[Jan]

The command audioread imports an MP3 or WAV file to a matrix, which contains the amplitudes. This is not the amplitude of the original signal, but it is exactly, what you want: The amplitude of the signal stored in the file.

Steganography in an MP3 file requires to de-compress the file at first, embed the secret information, and re-compress the signal to a new MP3 file. Note that the process of un- and re-compressing will degrade the quality of the signal already, even without inserting additional information.

[Rini Indrayani]

Actually, i had done steganography using homogenous frame from mp3 file. I had used audioread function to get the amplitude value to compare both of mp3 audio files (before and after steganography process). But i got the same value for all amplitudes from both mp3 audio files. So when i used that amplitudo values to calculate PSNR, the value is infinite. Because the difference value between both audio files is zero.that's why i am not sure about is it the true way to get amplitude or not.

[Star Strider]

My point, exactly.

Answer 2

[Star Strider]

The audioread function limits the amplitude of the output to ±1, so the original amplitude information will be lost unless you recorded a scaling value when you recorded it. You have to read the file in order to estimate the amplitude as a function of time.

[Rini Indrayani]

so there's nothing i can do with matlab to get the amplitude?

[Star Strider]

You cannot get the original amplitude. (This probably applies to all sound file formats and readers, not only MATLAB.) You can get the scaled amplitude variations, since they remain in the recorded signal. The problem is that you cannot even recover your original signal accurately if you use a format (such as MP3) that uses compression. Lost data can never be recovered.

I would never save scientific data (including physiologic and engineering data) as an audio file. They are not designed to record such data accurately (see Comparison of audio coding formats (link) and MP3 (link) for details).

You might be able to use the WAV (link) format without compression, but otherwise I would avoid using any audio recording format to store your data.

[Rini Indrayani]

I need to compare the mp3 audio quality before and after embedding of steganography process. I used mp3 file as the cover file. Then how if i do a conversion to both mp3 files (before and after steganography) to wav files? The conversion will be do the same decoding process to both audio files, right? Then can i finally be able to know the value of amplitude in that way?

[Star Strider]

I have not done steganography of audio files. Note that the compression process in the MP3 process may interfere with your steganography. I would use uncompressed WAV files.

[Rini Indrayani]

If I use WAV, is there any function in matlab to know the amplitude of that WAV file?

[Walter Roberson]

"If I use WAV, is there any function in matlab to know the amplitude of that WAV file?"

No.

[Walter Roberson]

You write asking about "amplitude", but you do not define what kind of amplitude you are talking about. Are you looking for Sound Pressure Level ?

"I need to compare the mp3 audio quality before and after embedding of steganography process."

Then why do you care about "amplitude" ? How are you measuring "audio quality" ? Is the measurement of audio quality something to do with Sound Pressure Level? Does it matter for your purposes whether the "amplitude" is a number from 0 to 1023, or a number from -512 to +511, or a number between -1 and +1 ? Is your quest for "audio quality" aided by knowing that the recording was a "line-in" level or at "mic level"?