Recreating high frequencies in dull-sounding music

Old recordings sound sometimes dull because they lack high frequencies. It is not possible to boost the high end with an equalizer, as there isn't anything to boost.

However, it is possible to synthesize artificial high frequencies into music. This makes the sound image a little bit more "open". However, you need to be careful with classical music, as added content doesn't always work with it. The technique works better in pop music.

There is a "Before" and an "After" sequence in the video clip. You can see more moving green bars at the right end of the spectrum in the "After" part.

Tape-recorder playback head azimuth

When a reel-to-reel tape or a Cassette tape is digitized, there is one crucial factor that affects to the sound quality. That is the vertical angle of the playback head gap in relation to the tape. This is called the Azimuth angle. If a tape is played back with the very same tape recorder that the tape was recorded with, the playback may be satisfactory. However, if a different tape recorder is used for digitizing, the sound is often muffled and dull.

Click picture for a larger image

The position of the playback head is not exactly the same as how the record head was. The record head leaves behind magnetic stripes in the ferric oxide. The orientation of these stripes should match as accurately as possible the direction of the gap in the playback head. We are talking about micrometers. If the playback head is sideways tilted with the stripes, the playback recorder doesn't get to use all the audio material that actually was recorded on the tape. High frequencies will be lost. A stereo recording gets phase errors and the stereo image gets wrong. If the recording has Dolby noise reduction, the noise reduction will operate incorrectly, causing the volume to pump.
The playback head must be adjusted so that it's orientation matches the invisible magnetic stripes on the tape before digitizing the tape. The adjustment must be done at this stage.

If a tape is digitized with a playback head that has an azimuth error, it is not possible to correct this later. We adjust each tape side azimuth separately.

Below are two different visualizations of the same audio clip. The first is a spectrum analyzer display. Low frequencies are at the left, high frequencies on the right. You will notice more green bars at the right hand side of the window in the "After" section, as the azimuth has been adjusted to optimum.

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