Need advice regarding editing an MP3 file in Sonar

MP3 encoding is a destructive form of representing audio. This is important to know as any audio brought into Sonar is converted on the fly into a .wav file. Sonar then performs any editing and on conversion to MP3 is again compressed by deleting "unnecessary" data. That means that continuous importing editing and exporting of the same mp3 file will compromise the quality more and more with each cycle.

You select bit depth and/or dither type on export in Sonars export dialog.
Bit depth conversion requires dither.
Sample rate reduction eg 48khz to 44.1khz does not require dither.
Mp3s are generally 16 bit but if you're recording in 24 bit and going back to MP3 then the MP3 conversion will take care of any bit rate reduction and sample rate conversion (SRC). This is all done when you select the bitrate for your intended MP3 file.
When using Sonars Burn to CD feature you are not creating mp3s, rather creating an audio CD which is different..

Mp3s are stored for replay on data CDs, that can store many more tracks than and ordinary audio CD. You need to tell Sonar which type of CD you are going to burn.

I import all files in at 32-bit float (64-bit engine) and 48000Hz, although if a file is a 16-bit render it might be a pointless exercise if it's for burning CD. Adding dither will add discrete noise to the recording, so it won't be beneficial if the file is already 16-bit.

The audio will be 16 bit. Thank you Mudgel that’s what I needed to know.

Actually no, if Sonar is in 24 bit mode any files you export will be 24 bit by default. HOWEVER if you use the built in CD burner, it will automatically select 16 bit because that is the only thing that goes on an audio CD.

On the whole I don't think you need to worry very much, these differences are very minute in the best of circumstances and you're already starting with an MP3.

EDIT: by the way, unless you tick the box "import at original bit depth" Sonar will automatically upscale the mp3 to 24 bit during import (since that's what your project is set to), and create internal bounces at 32 bit floating point. This is actually beneficial as it reduces possible calculation errors by a factor of thousands. Downscaling at the final step is better than doing all operations on a 16 bit file.

There's quite a bit of evidence that most people can't distinguish between different dithering algorithms or even tell if dithering has been applied or not.
 
All converting 16 bit to 24 bit (or more) does is add packing zeros to make up the difference in bit depth. After processing, going the other way involves making decisions about dithering. I suggest exporting the 16/44.1 wav that will go on to the CD several times using a different dither each time. Then pick the one that sounds best (if you can tell them apart).
 
The quality of the final product will depend far more on the quality of the original mp3 than anything Sonar does to it in the way of bit depth or sample rate conversion.

.

I wrote a post to try to summarise this thread but happened to go back to your first post. And subsequently replaced it with this

For anybody following this thread I bet they are confused. I've just gone back to your first post and you've heavily edited it so that the following answers pretty much don't make sense anymore. Had we been given that information first your real question could have been answered in a few words.

In future perhaps just leave previous posts because not only are you getting an answer but anyone who follows along can get answers too. Now that's no longer the case.

Anyway I'm out a here now.

OK then I'm obviously having a brain disturbance of some sort My apologies then. What?
 
Until my previous post I have been using my ipad (just updated to ios 8.3 yesterday) to work on this thread and scour the forum generally. I have been having all sorts of screen issues which for whatever reason didn't show me the last part of your initial post until earlier when I opened my PC to use Sonar and also checked this and a few other threads.
here's what I saw.
 

Dilaco1I have an atmospheric soundtrack in MP3 format. I want to extend its duration, and then burn it to CD, but I need to do so with as little alteration to the original audio as possible – i.e. without dithering, rendering, decoding, etc. any more than I have to.
 
I don’t know much about MP3s but I tried importing the MP3 to Sonar (Import bit depth setting: “Original”), then copied the audio clip and pasted it on to the end of itself – doubling the audio’s duration. Then I tried using Sonar’s Burn to CD function. When I clicked on its Add button to bring up the audio file, it only added the original audio and not the cloned audio. So I thought of doing a “Bounce to Clips” to render the two clips together. The problem is that this bouncing is at the wrong bit depth –32 bit. I thought of changing my rendering setting to 16 bit was concerned it would be low quality/accuracy.

 
Hence my comment that if you had posted the additional information yadayada and carry on about MP3s. I'm a pretty thorough reader and usually read all posts in threads and all the posts.
 
the main thing is are you clear now on what to do?
 
if you use 32 and or 64 bit float for internal processing then you don't need to dither for that reason alone.
If you go up in bit depth. process then go down in bit depth you dither as your last stage in processing which you already know by your comments in post #13. BUT there is some discussion on dither which suggests that the kind of material itself would decide on the use of dither when reducing bit depth. Higher frequency more likely need dither than low frequency material. Its one of those long ongoing debates with no winners I would reckon. You can always experiment to see which you think sounds better. Dither or don't and the choice of dithers  to use.
 
In the meantime I've had a look at Sonar's CD burner tool and didn't realise that it required exporting files in the first place. BUT in any case it clearly states in the online help for the CD Burner that even if a file that is not at 16 bit is selected, the burner software will take care of that in the whole process of uploading (to the program) and burning the files to CD.
 
And as sander said by default Sonar will only spit out files at the bit depth to which it is set in the preferences section he described, I actually specified making those various selections because the export dialog is that comprehensive that you may select all manner of things including bit depth etc.
 
I hope that's clear now.
 
 
 

All I meant is that Sonar, by default, will be set to export a file at the project bit depth. You can change this easily. But the fact that you, at one point, dragged a 16 bit file into the project doesn't change this behavior.

dilaco1, you may be over-thinking the process and looking for options that just don't exist.
 
Leave your SONAR settings at their defaults. SONAR is going to convert the MP3 into a 32-bit wave file when you import it. There won't be any modifications to the file beyond the decoding process. Quality will depend almost entirely on the bitrate of the original file and unfortunately there's nothing you can do to improve it. 
 
Of course, when you ultimately export the final file it will be truncated to 16 bits and have dither noise added. But that's required for CDs so there's no point in fretting about whether you could have done something better. As long as the original MP3 was decent quality, chances are you'll not hear much difference between the original and the final CD version.

Yes.

In Preferences/files/Audio Data make sure that for Import Audio, Original is selected. This is Sonars default

With the above setting at default (Original), any imported clip will retain its original bit depth on export.

If you want however you can change this behavior and select 16, 24, 32, 64 for bit depth, perhaps in line with your project bit depth.