How to identify transcoded files

[martzar]

Newbie here, so I could use all the help I can get. If I were to download a music file off the internet (blog, website, etc) how can I check to see that the files are not transcoded? I know I will have to open each song in Audacity and choose Spectrum under the drop down menu, but that is all I know. Also, I believe I need to view the file frequencies between 14kHz and 17kHz. Please help! Thanks, Addison.

[bgravato]

What do you mean by transcoded files? And what's the relation of that with frequency analysis?

The term transcoding usually refers to converting one digital format to another (different) digital format: http://en.wikipedia.org/wiki/Transcoding
Whether that will imply a change in the frequency spectrum or not depends on the formats involved, transcoding options, bitrates, etc.

Can you explain in other words what you're looking for?

[martzar]

Exactly, I want to see if the files I downloaded were transcoded or not. Usually when a file is transcoded (especially over the internet) it effects the quality. For instance, if the file is a perfect FLAC, it will have 20,500 HZ (making it flawless), while a CBR 320 MP3 only contains 18,000 HZ. I need to figure out how I can view the spectrums of the songs to see how many HZ they have, which in turn will tell me if they were transcoded or not.

[bgravato]

To plot a spectrum in Audacity (1.3.12) select the part of the track you want to analyze and then go to the menu Analyze and select Plot Spectrum...

But that's not a very accurate way of analyzing what you propose... Frequency response will depend mostly on the audio in the file... If it's a bass only recording you won't have high frequencies regardless of the codec used.

Even if it's a track with high pitched sounds, depending on the way it was recorded, equipment used, post processing, etc... you may or may not have frequencies above 18kHz (or whatever the threshold might be).

[martzar]

That's a very good point. Thanks for answering, but now I'm more intrigued about learning a better and more accurate way since there's too many variables in my original procedure. What do you think is the best way?

[steve]

If the file was carefully transcoded between lossless formats, there will be no way of knowing as the sound quality will be virtually identical to the original.
The important part is how it sounds. A first generation conversion from the original to another format can be bad. A second or third generation conversion could be very much better, depending on how it is done, so the sound quality (what it sounds like) is of much greater importance than how it got there.

[billw58]

I don't understand how this can be an issue at all. If you download a FLAC file from the internet and it shows up on your computer as a FLAC file then it was not transcoded during delivery. Right? The wikipedia article talks about, for example, reducing the resolution and colour depth of images sent between mobile phones. I have never heard of a FLAC file being encoded to MP3 (for example) for transmission (to reduce bandwidth?) then re-encoded as FLAC after it arrives on your computer.

-- Bill

[martzar]

FLAC was merely an example, but you can definitely transcode FLAC to mp3 because it's only a single lossy stage. I do it all the time with a program called XLD (for mac). If I download a mp3 from the internet, I want to know what kind of mp3 it is (what the bitrate is) so I can see if it went through multiple lossy encodes. "Whenever you encode a file to a lossy format (such as MP3, M4A (AAC), Ogg Vorbis, 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, 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." So if i were to download an advertised CBR 320 mp3 off the internet, I want to make sure it's truly that advertised quality because if it was transcoded from, say a VBR v0 mp3, it wouldn't actually have the quality of a CBR 320 as advertised. FLAC to any mp3 is fine, but mp3 to mp3 isn't. I just need a suggestion of how I can check the quality of mp3 I got from the internet.

[bgravato]

I just need a suggestion of how I can check the quality of mp3 I got from the internet.

In a simple answer.. you don't! I don't think there's any way you can tell how many times a file has been reencoded. Yes each reencoding will make more damage, but I don't think there is any (simple) mathematical analysis you can do to predict that...

You can judge the quality by hearing, which is a very subjective way of analysing it, but probably the most sensitive...

It also depends a lot on the source material... I've listened to original cds which had some clipping on it... So it's hard to tell where the damage comes from, unless you have the original to compare it too...

[martzar]

If there is a way to view the bitrates of each song, there's a way to see if it's been transcoded multiple times. Say i downloaded a cbr 320 mp3 from the internet, i check it out on audacity to see how many hz it has. A reg 320 would have 18,000 hz, so if i see it only has 16,000 hz, i know it's been transcoded at least two times (from flac to 320 to 128 kbps, which has 16,000hz).