I noticed while listening to the Patreon-only feed ($5 / month contributor) from Daily Tech News Show (DTNS) that the volume would be quiet in some places, and loud in others. This was most noticeable on Wednesdays when Scott Johnson would be on the show with Tom Merritt. Scott would be really loud on the audio, and Tom would be really quiet.
Since the stream is coming directly from Alpha Geek Radio, there is little that can be done to adjust the levels of the stream as it is recorded.
I wrote this script to download the RSS, and run the sox compand command on it to adjust the amplitude levels to help me hear the audio while commuting in my car. After converting the audio, the "normalized" files are uploaded and hosted from an S3 bucket configured to host them via HTML.
- LAME - This script assumes LAME is is in the path when the script is run.
- SOX - This script assumes that SOX is also in the path when it is run.
- Python Requests module - For downloading the RSS and MP3 files
- PyOpenSSL libraries for securely requesting resources from S3
- Boto3 library for uploading new files to S3 - this also assumes that if you're developing this locally, you have the AWS environment variables and sufficient access to S3.
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Download RSS You'll need to set the DTNS_FEED_URL environment variable for this to work. Since this is a pay-for feature of the show, I did not want to include my link in this repository
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Decode MP3 to WAV The feed includes only MP3 files of the show. The sox command will only operate on WAV files, so the MP3s need to be decoded before sox can be run. It uses the following format to decode the mp3:
lame --decode <mp3fie> -
Execute SOX compand (compress / expand) operation Options used in this script were taken directly from the documentation for noisy car listening to soft music. Specifically:
compand 0.3,1 6:−70,−60,−20 −5 −90 0.2See Below for more detail about the compand command -
Re-encode the WAV to MP3 for upload and hosting. Arguments:
lame -b 96 <fileName> -
Copy the new MP3 to S3
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Replace existing DTNS Feed URL list with URLs from normalized S3 bucket.
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Clean-up any local files created during the conversion.
From the sox documentation:
compand
attack1,decay1{,attack2,decay2} [soft-knee-dB:]in-dB1[,out-dB1]{,in-dB2,out-dB2} [gain [initial-volume-dB [delay]]]Compand (compress or expand) the dynamic range of the audio. The attack and decay parameters (in seconds) determine the time over which the instantaneous level of the input signal is averaged to determine its volume; attacks refer to increases in
volume and decays refer to decreases. For most situations, the attack time (response to the music getting louder) should be shorter than the decay time because the human ear is more sensitive to sudden loud music than sudden soft music. Where more than one pair of attack/decay parameters are specified, each input channel is companded separately and the number of pairs must agree with the number of input channels. Typical values are 0.3,0.8 seconds.
The second parameter is a list of points on the compander’s transfer function specified in dB relative to the maximum possible signal amplitude. The input values must be in a strictly increasing order but the transfer function does not have to be monotonically rising. If omitted, the value of out-dB1 defaults to the same value as in-dB1; levels below in-dB1 are not companded (but may have gain applied to them). The point 0,0 is assumed but may be overridden (by 0,out-dBn). If the list is preceded by a soft-knee-dB value, then the points at where adjacent line segments on the transfer function meet will be rounded by the amount given. Typical values for the transfer function are 6:−70,−60,−20.
The third (optional) parameter is an additional gain in dB to be applied at all points on the transfer function and allows easy adjustment of the overall gain.
The fourth (optional) parameter is an initial level to be assumed for each channel when companding starts. This permits the user to supply a nominal level initially, so that, for example, a very large gain is not applied to initial signal levels before the companding action has begun to operate: it is quite probable that in such an event, the output would be severely clipped while the compander gain properly adjusts itself. A typical value (for audio which is initially quiet) is −90 dB.
The fifth (optional) parameter is a delay in seconds. The input signal is analysed immediately to control the compander, but it is delayed before being fed to the volume adjuster. Specifying a delay approximately equal to the attack/decay times allows the compander to effectively operate in a ‘predictive’ rather than a reactive mode. A typical value is 0.2 seconds.
The following example might be used to make a piece of music with both quiet and loud passages suitable for listening to in a noisy environment such as a moving vehicle:
sox asz.wav asz-car.wav compand 0.3,1 6:−70,−60,−20 −5 −90 0.2The transfer function (‘6:−70,...’) says that very soft sounds (below −70dB) will remain unchanged. This will stop the compander from boosting the volume on ‘silent’ passages such as between movements. However, sounds in the range −60dB to 0dB (maximum volume) will be boosted so
that the 60dB dynamic range of the original music will be compressed 3-to-1 into a 20dB range, which is wide enough to enjoy the music but narrow enough to get around the road noise.
The
6:selects 6dB soft-knee companding. The −5 (dB) output gain is needed to avoid clipping (the number is inexact, and was derived by experimentation). The −90 (dB) for the initial volume will work fine for a clip that starts with near silence, and the delay of 0.2 (seconds) has the effect of causing the compander to react a bit more quickly to sudden volume changes.