If there is no video signal to the VCR input, most VCRs will record black video and generate a control track while the sound is being recorded. Some early VCRs record audio without a control track signal; this is of little use, because the absence of a signal from the control track means that the linear tape speed is irregular during playback. More sophisticated VCRs offer stereo audio recording and playback. Linear stereo fits two independent channels in the same space as the original mono audiotrack. While this approach preserves acceptable backward compatibility with monoaural audio heads, the splitting of the audio track degraded the signal's SNR, causing objectionable tape hiss at normal listening volume. To counteract the hiss, VCRs use Dolby B noise reduction for recording and playback. This dynamically boosts the mid-frequency band of the audio program on the recorded medium, improving its signal strength relative to the tape's background noise floor, then attenuates the mid-band during playback. Dolby B is not a transparent process, and Dolby-encoded program material exhibits an unnatural mid-range emphasis when played on VCRs that are not made to work with this form of noise reduction. Another option is to use noise reduction similar to that of DBX—that is, by recording with high volume but with a compressed dynamic range. On playback the decompression will yield original audio; because weaker signals are attenuated, the hiss can also be significantly attenuated. [Why is this mentioned? Has this been done, or is somebody just brainstorming at Wikipedia?] High-end consumer recorders take advantage of the linear nature of the audio track, as the audio track could be erased and recorded without disturbing the video portion of the recorded signal. Hence, "audio dubbing" and "video dubbing", where either the audio or video are re-recorded on tape (without disturbing the other), were supported features on prosumer linear video editing-decks. Without dubbing capability, an audio or video edit could not be done in-place on master cassette, and requires the editing output be captured to another tape, incurring generational loss. Studio film releases began to emerge with linear stereo audiotracks in 1982. From that point onward nearly every home video release by Hollywood featured a Dolby-encoded linear stereo audiotrack. However, linear stereo was never popular with equipment makers or consumers. Another linear "control" track, at the tape's lower edge, holds pulses that mark the beginning of every frame of video; these are used to fine-tune the tape speed during playback, so that the high speed rotating heads remained exactly on their helical tracks rather than somewhere between two adjacent tracks (known as "tracking").