I'm working on a small video editing project at home and am new at this. I have a multiple videos that I'm combining together into one video and am using the audio from one of these videos as the single audio track.
The video that contains the audio is an MP4 with the following attributes (from MediaInfo):
Audio
ID : 2
Format : AAC
Format/Info : Advanced Audio Codec
Format profile : LC
Codec ID : 40
Duration : 4mn 14s
Bit rate mode : Variable
Bit rate : 152 Kbps
Maximum bit rate : 178 Kbps
Channel(s) : 2 channels
Channel positions : Front: L R
Sampling rate : 44.1 KHz
Compression mode : Lossy
Stream size : 4.62 MiB (2%)
Title : IsoMedia File Produced by Google, 5-11-2011
Encoded date : UTC 2011-08-17 04:52:52
Tagged date : UTC 2011-08-17 04:52:53
Using Corel Video Studio Pro X4, I've been rendering my video project to an MP4 file using the best standard setting (1920 x 1080, etc) . However, the new video's audio has very slight crackling and popping sounds intermittently throughout the track that did not exist in the original. At first I did not even notice it, but after watching through the generated video a few times I started to notice the degradation.
When I checked the format of the generated video file I noticed that the audio attributes were different (see below), specifically, the sampling rate and bit rate.
Audio
ID : 2
Format : AAC
Format/Info : Advanced Audio Codec
Format profile : LC
Codec ID : 40
Duration : 4mn 11s
Bit rate mode : Constant
Bit rate : 128 Kbps
Channel(s) : 2 channels
Channel positions : Front: L R
Sampling rate : 32.0 KHz
Compression mode : Lossy
Stream size : 3.84 MiB (1%)
Language : English
Encoded date : UTC 2012-01-14 05:21:58
Tagged date : UTC 2012-01-14 05:21:58
I tried regenerating the video file using the following audio settings, still in MP4 format and still got degraded audio, though noticeably worse.
Audio
ID : 2
Format : AAC
Format/Info : Advanced Audio Codec
Format profile : LC
Codec ID : 40
Duration : 4mn 11s
Duration_FirstFrame : -21ms
Bit rate mode : Constant
Bit rate : 256 Kbps
Channel(s) : 2 channels
Channel positions : Front: L R
Sampling rate : 48.0 KHz
Compression mode : Lossy
Stream size : 7.68 MiB (2%)
Language : English
Encoded date : UTC 2012-01-14 16:48:04
Tagged date : UTC 2012-01-14 16:48:04
Finally, I regenerated the video with these settings (same sampling rate as the original file and the closest bit rate setting available) and the audio now sounds the same quality as the original.
Audio
ID : 2
Format : AAC
Format/Info : Advanced Audio Codec
Format profile : LC
Codec ID : 40
Duration : 4mn 11s
Duration_FirstFrame : -23ms
Bit rate mode : Constant
Bit rate : 160 Kbps
Channel(s) : 2 channels
Channel positions : Front: L R
Sampling rate : 44.1 KHz
Compression mode : Lossy
Stream size : 4.80 MiB (1%)
Language : English
Encoded date : UTC 2012-01-14 16:51:35
Tagged date : UTC 2012-01-14 16:51:35
Is it normal for audio to degrade like this when output using different settings? I would assume that it would be normal for sound to be of less quality with lower settings, but not quite like this. Thoughts?
Could this be a defect in the software that I'm using, or is bit rates and sampling rates something I have to be aware of when doing video editing? I had hoped that this would be all taken care of by video editing software so that I didn't need to think too much about.
I've used mp4box to extract the audio tracks from the video files and have uploaded them here. I've included the original 44.1 KHz audio track and the tracks from two of the rendered video files with degraded quality (32KHz & 48KHz).