The SL-HF100UB was Sony's only domestic Betamax machine in the UK equipped with Beta hi-fi stereo sound. (Hi-fi sound was added to some Sony SuperBeta models as well as a few high-end Sanyo decks.) Beta hi-fi PAL uses two extra heads mounted on the video head disk to record stereo sound on the video track. Because of the very high writing speed of the video heads, the sound quality of Beta hi-fi is very good - comparable to CD quality. And, more than 3 hours of continuous recording is possible on a Betamax cassette, compared with only 74 minutes on a CD.
The Beta hi-fi PAL sound system is made possible by a strange property of magnetic tape. When something is recorded on tape, the higher frequencies are stored near the tape's surface, whereas lower frequencies are stored deeper inside the layer of magnetic particles on the tape. Since video signals are high frequency and sound is relatively low frequency, the designers found that it was possible to record both signals one on top of another; the video signals stay close to the surface of the tape while the hi-fi audio stays deeper beneath the surface. When playing back the recording, it's easy for the machine to separate the video and hi-fi sound signals coming from the tape.
A standard Betamax player without the Beta hi-fi heads simply does not 'see' the hi-fi sound signals, and plays the picture back perfectly without any hi-fi sound. To be backwards-compatible, the SL-HF100UB includes a normal mono audio head, so its recordings can be played back with normal quality mono sound in other Betamax machines. Likewise, the SL-HF100UB can play non Beta hi-fi recordings.As well as recording hi-fi sound and pictures together, the SL-HF100UB can be used to record just sound only, or record the picture from the TV with sound from an external source such as an FM tuner or (nowadays) a NICAM stereo decoder. Stereo TV broadcasting hadn't started in the UK when this Betamax was launched. In those days, it was common for the BBC to broadcast live music concerts in mono on TV and simultaneously in stereo on FM radio. The 'Simulcast' button on the SL-HF100UB enabled the TV picture to be recorded together with the stereo sound from the radio broadcast. It is even possible to set the timer to record a Simulcast or just the sound of your favourite radio show to listen to later in high quality Beta hi-fi stereo.