Why the noise when boosting volume?

[Djard]

Some video files have such weak volume that you need a 50,000 watt RMS amp to hear the sound. OK, I'm exaggerating a little.
I'm able to recode such video and correct the volume; but when I do, I often--not always--also get an awful amount of indistinct rumbling noise in the background, which only surfaces during silent sections. The videos to which I refer are not pirated cam movies.

The noise that appears when boosting weak video audio reminds me of those recordings on cheap tape recorders that had automatic gain: the closer you got to the microphone, the built in limiter would attenuate the sound; if you spoke into the microphone further back from the mic, the audio would be amplified, picking up all the ambient noise.

I'm thinking of demuxing weak audio and using Audacity to increase the volume, hoping to filter out the noise with the equalizer. But that's a lot or work. I began using ConvertXtoDVD to boost the volume, after the "Volume Correction" feature in XMediaREcode ceased to work; so I don't know if ConvertX is the problem. Has anybody found a clean way to correct weak volume in video files?

[Cornucopia]

Filtering, whether by equalization, gating/companding, or other means, are a few ways that one can SOMEWHAT remove noise after-the-fact.

But the truth remains that noise that arises when doing an original record/mix pretty much stays with the signal forever. It IS part of the signal, and only methods that can "isolate" it away from the signal can do anything to make any difference. And since most methods are far from perfect, the isolation that is required is also far from perfect.

The only true remedy is what professionals have recommended all along: don't let it become part of the signal in the first place. Use mikes & pre-amps with VERY LOW self-noise; use proper gain structure (in both analog & digital realms); monitor using calibrated output; physically isolate your signal from your noise via pickup pattern and placement and with obstructing/dampening materials; work in an extremely low-noise environment.

Usually, once you're at the stage you are at now, there is little you can do. Attempts to make strong changes often end up removing some of the wanted signal along with the noise.

Scott

[davexnet]

Perhaps a small sample of the source to show the problem....

[AlanHK]

I'm thinking of demuxing weak audio and using Audacity to increase the volume, hoping to filter out the noise with the equalizer.

Audacity has a noise reduction "effect".
You train it by selecting a section of noise and a strength and it tries to remove that from the track.

Not perfect, may cause odd side effects, but worth a try in moderation.

[Djard]

I solved my problem by accidentally deleting the file. Go me!

But thanks for the excellent feedback. Now I'm curious and eager to try the noise reduction filter in Audacity.

Since the noise to which I refer unpleasantly swells only during silent parts of the audio, it should be possible to attenuate those parts in Audacity, without affecting the wanted sections. But again, such work sounds laborious. I may try that next time I encounter the problem.

Cheers!

[pandy]

Audacity have quite simple noise filter, more advanced is avaiable in Adobe Audition and even more complex are avaiable as a separate plugins: iZotope family

Real-time similar to Audacity noise reduction is available in FFDSHOW.

[Djard]

Thanks. I'll add that to my arsenal of multimedia tools.

[hello_hello]

Since the noise to which I refer unpleasantly swells only during silent parts of the audio, it should be possible to attenuate those parts in Audacity, without affecting the wanted sections. But again, such work sounds laborious. I may try that next time I encounter the problem.

Sounds to me like the media player or converter you're using employs some sort of compression or level normalizing which effectively increases the volume of the quieter parts, and what would normally be almost inaudible background noise sometimes becomes loader and sounds unnaturally noisy. When the audio is only sparse (say during sections where there's just a bit of dialogue) the player or converter mightn't turn the volume up and down fast enough for the background noise to become noticeably loud, or sometimes you might hear a "pumping" effect as the audio is turned up and down. I'm not sure if the converters you're using employ some sort of volume leveling (compression) but if they do you may be getting a double whammy..... leveling on both conversion and playback.

If that's the case then you may be able to dial the compression down a little when encoding, or disabling it completely may fix the problem, but what you're describing sounds to me like it might be just the result of using volume leveling (when encoding or on playback, or both) and the quieter sections of some audio tracks just sound noisier than others. It may be possible the volume leveling you're using is actually causing the problem you're wanting to fix.
Usually when encoding programs "normalize" the volume they simply increase it until the peaks are at 0db, so the entire audio is increased by the same amount, but your programs may be attempting to do something more clever. They may be capable of both "normalizing" and "leveling", in which case it might pay to only use the former. They could also be applying AC3 dynamic range compression when converting (if the original audio is AC3) which may have a similar effect.

I generally use fairly heavy compression on playback (I use a WinAmp plugin with ffdshow) and while I've managed to set it up so the compression is inaudible 99% of the time, occasionally I do hear the effect you describing, even when playing good quality audio soundtracks. It's just one of those compromises resulting from the use of compression. I never use leveling when encoding though (just peak normalization) so the relative audio levels are only changed on playback.

[Djard]

You've given me much to think about. I'm going to put on my detective's hat and do some sleuthing; since the problem I described (ambient noise swells during silent parts) occurs inconsistently when I configure ConvertXtoDVD to increase the volume of the audio, which then includes normalization, even with a custom setting. Maybe the problem is exclusive to AC3.

[Hoser Rob]

If you used volume normalization in your dvd player rather than when encoding it'd probably work better.

[hello_hello]

If, as it sounds, the problem only occurs when converting with ConvertXtoDVD..... well from the manual......

"The new sound boosting scheme is in fact a function that may both amplify or attenuate the sound. It's more complex than a simple sound normalization or amplification, but the result is way much better. The sound level is constantly adjusted to reflect the human perception of how loud or soft is a sound. So it gives an output sound pressure (target audio level) ranging from -30 to -3 dB. If the input sound is louder than the the target sound it is attenuated; if it's softer it is amplified."

Maybe when the say the result is much better, they're actually using some little known meaning of the word "better" which actually means "worse"?
It sounds to me like it's ConvertxToDVD causing the problem, and there doesn't seem to be a way to tell it to perform simple peak normalization only. I haven't created an actual DVD video disc in years so I couldn't even try to explain how to adjust the audio manually after the video has been converted, however depending on the type of video you're converting to DVD, the easiest solution may be to manually convert the original audio to the desired format first (if you need to increase the volume) while performing simple peak level normalization. You could then replace the original audio with your converted version and according to the manual, if it's in a DVD compatible format ConvertXtoDVD will simply copy it if the audio setting is set to "Automatic". Otherwise if the original audio doesn't need to have it's volume increased you can simply disable the volume normalization.

As I've no idea what sort of files you're generally trying to convert to DVD it's not really possible to explain how to manually convert/normalize the audio before it's all converted to DVD, but depending on the original format, chances are it won't be hard to do once you have the appropriate tools installed. For instance extracting audio from MKV files, converting it to another format, and then using the newly encoded audio to replace the original is something I do fairly regularly.