Audio Cleanup

When you capture the audio of a VHS tape with the process outlined here, it is likely to be noticeably noisy. In addition to the noise contained in the VHS and generated by the VCR (see the box titled "Removing Audio Noise from Old VHS Tapes" ), the capture card also produces its own noise. Especially true when using cheap capture devices, the capture card will always introduce a barely audible buzzing sound to the resulting recording.

To mitigate the noise introduced by the capture card, you can use SoX. First, capture an audio segment that only contains the noise introduced by the capture card. With your EasyCAP connected to the VCR when it is not generating any sound output (e.g., when the tape is paused), capture a few seconds of an audio segment as follows:

gst-launch-1.0 -q alsasrc device=$alsadevice ! wavenc ! fdsink | sox -t wav - -n trim 0 1 noiseprof noiseprofile

This will create a file named noiseprofile that contains a description of the noise introduced by EasyCAP. This profile can be used for reducing the noise in the audio you capture.

Now, the tape's digitized audio can be extracted from its file and cleaned with:

ffmpeg -i vhs.mkv -acodec pcm_s16le -vn vhs_tmpaud.wav
sox vhs_tmpaud.wav vhs_tmpaud-clean.wav noisered general_noise.prof 0.21

A clean version of the audio will be recorded to vhs_tmpaud-clean.wav. The last value of the sox command above defines the denoiser's aggressiveness. Higher values mean more noise reduction, but at the expense of sound fidelity. Denoising is a destructive procedure that may wipe information away. Values that are too high will cause audible sound artifacts that are in fact worse than the original noise. Values between 0.21 and 0.31 are reasonable.

Finally, the audio is muxed back with the video stream. In this example, a video-only stream is extracted from vhs.mkv and stored in vhs_video_only.mkv. Then this video stream is combined with the clean version of the audio just generated, and the whole file is stored as vhs_whole.mkv:

ffmpeg -i vhs.mkv -vcodec copy -an vhs_video_only.mkv
ffmpeg -i vhs_video_only.mkv -i vhs_tmpaud-clean.wav -map 0:v -map 1:a -c:v copy -c:a copy vhs_whole.mkv

The Transcoding Process

You now have a video file with clean audio that uses about 15GB of disk space. This file may be transcoded into some more practical format using any encoder, such as HandBrake or MEncoder. However, I used FFmpeg in my example, because it is easily available and well documented.

For the excess video usually found at the beginning and end of a recording, trim it out with the following command:

ffmpeg -i vhs_whole.mkv -acodec copy -vcodec copy -ss $start_position -to $end_position vhs_trim.mkv

Start and end positions are values in the form hours:minutes:seconds, such as 01:30:50.

Listing 3 transcodes the captured video file and the cleaned audio file into vhs_final.mkv.

ffmpeg -i vhs_trim.mkv
       -vf "crop=(iw-10):(ih-14):3:0,pad=iw+10:ih+14:(ow-iw)/2:(oh-ih)/2,hqdn3d=2:1:2:3"
       -c:v libx264 -flags +ilme+ildct -profile:v high -tune:v animation
       -preset veryslow -crf 26 -c:a libfdk_aac -b:a 224k -f matroska vhs_final.mkv

The second line in Listing 3 is a set of video filters that are applied to the encoding; crop removes the overscan at the picture's borders, and pad replaces it with simple black bands. hqdn3d is a video denoiser (see the "Removing Video Noise" box).

Figure 3: Before (top) and after (bottom) the application of an aggressive denoiser.

The veryslow H.264 preset ensures that the best quality-to-size ratio is obtained at the expense of encoding time (for other presets, see Table 4). The ilme and ildct flags ensure that interlacing information is preserved. The tune:v option serves to adjust the H.264 encoder to the content type being transcoded; I used animation in Listing 3 (for other tune options, see Table 5). libfdk_aac encodes the sound into Advance Audio Coding (AAC) at 224Kbps. (See also the "Patent-Encumbered Codecs" box.)

The profile:v switch selects an encoding profile. Files encoded with a high profile setting will be playable by most modern multimedia appliances. The baseline profile may be used in order to ensure compatibility with older appliances, at the expense of compression efficency. [5]

Wrapping Up

The last thing you need to do is provide (optional) metadata to the file. Listing 4 shows an example metadata.xml file. Matroska metadata is not well documented, but if you want to delve deeper, see some examples online [7].

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE Tags SYSTEM "matroskatags.dtd">
<Tags>
  <!-- movie -->
  <Tag>
    <Targets>
      <TargetTypeValue>50</TargetTypeValue>
    </Targets>
    <Simple>
      <Name>TITLE</Name>
      <String>Happy Rottweiler</String>
    </Simple>
    <Simple>
      <Name>DIRECTOR</Name>
      <String>Rubén Llorente</String>
    </Simple>
    <Simple>
      <Name>DATE_RELEASED</Name>
      <String>1992</String>
    </Simple>
    <Simple>
      <Name>ORIGINAL_MEDIA_TYPE</Name>
      <String>VHS</String>
    </Simple>
  </Tag>
</Tags>

The final step is to merge the metadata with the Matroska file, as follows:

mkvpropedit "vhs_final.mkv" --edit info --set "title=Happy Rottweiler" -t global:metadata.xml --edit track:a1 --set "language=spa" --set "flag-default=1"

This command sets the file's title Happy Rottweiler, merges the metadata file metadata.xml with the vhs_final.mkv file, and sets the audio track number 1 language value to Spanish. The only audio track is set as the default audio track.