Hi friends,
I recently pulled the PCM audio from my copy of The Visitors by ABBA, 1982 on Polydor into Audacity and made an interesting discovery.
Do you see the very faint tone at about 16kHz? (right-click and select open image in new tab)
Each track on the disc display this anomaly. A high frequency tone or noise in the background of the music. The level is quite low so it is not audible, but it can be clearly seen. Consider that this is a view of the actual PCM samples on the disc. At no time did I pass any of these signals through an additional stage of ADC before viewing them in Audacity.
There is a simple explanation for the presence of this noise or tone. Way back in the early 1980s the only way to master a CD was to write the digital data as a pseudo video signal to U-Matic video tape. Before the audio data could be written to the video tape it had to be processed by a PCM Processor. It converted the digital audio into a video signal the video recorder could record. It would appear the audio was played into the ADC units of this PCM processor (typically a Sony PCM-1630) and in the process the horizontal scan frequency of the video signal leaked in. The horizontal scan frequency is 15.750 or 15.625kHz depending on whether NTSC or PAL video timing is used during mastering. When you look at the spectrogram view you will see the noise/tone at that location.
Here is Highlights from the Phantom Of The Opera, 1987 on Polydor
It is very likely that many other early CDs display this interesting anomaly. I'm also curious if it is only Polydor. Go have a look and report back.
Cheers!
I recently pulled the PCM audio from my copy of The Visitors by ABBA, 1982 on Polydor into Audacity and made an interesting discovery.
Do you see the very faint tone at about 16kHz? (right-click and select open image in new tab)
Each track on the disc display this anomaly. A high frequency tone or noise in the background of the music. The level is quite low so it is not audible, but it can be clearly seen. Consider that this is a view of the actual PCM samples on the disc. At no time did I pass any of these signals through an additional stage of ADC before viewing them in Audacity.
There is a simple explanation for the presence of this noise or tone. Way back in the early 1980s the only way to master a CD was to write the digital data as a pseudo video signal to U-Matic video tape. Before the audio data could be written to the video tape it had to be processed by a PCM Processor. It converted the digital audio into a video signal the video recorder could record. It would appear the audio was played into the ADC units of this PCM processor (typically a Sony PCM-1630) and in the process the horizontal scan frequency of the video signal leaked in. The horizontal scan frequency is 15.750 or 15.625kHz depending on whether NTSC or PAL video timing is used during mastering. When you look at the spectrogram view you will see the noise/tone at that location.
Here is Highlights from the Phantom Of The Opera, 1987 on Polydor
It is very likely that many other early CDs display this interesting anomaly. I'm also curious if it is only Polydor. Go have a look and report back.
Cheers!
