VHS Playback Physics Simulator :: Transdimensional PhoenixVHS Playback Physics Simulator<br>2026-07-23<br>#videomachine<br>I want to study my centroid methodology more conveniently and later prove formally that what I came up with actually results in aligned VCRs. So I have been working on a physics model of the VHS playback - specifically the impact of mechanical aspects of playback on the RF envelope read by video heads.<br>» The VHS playback simulator can be accessed here «

I&rsquo;m not that good at HTML, so it&rsquo;s just kludged together for the most part&mldr; but I think I fixed the most embarrassing HTML-related bugs by now.<br>This is effectively a purely geometric model with just basic physics - a very simple, idealistic reflection of what happens inside the VHS playback. The simulator calculates the following things:<br>How geometry of tape wraps around the drum<br>How the video heads pass over video tracks on the tape during rotation<br>The basic operation of the servo circuit and how CTL pulses are used<br>Simple model of the audio/control head that reflects how X-value, azimuth and tilt all impact the output signals<br>Simple model of linear and Hi-Fi audio which shows phase relationship between the two depending on the adjustments of the A/C head<br>Basic physics of azimuth rejection that ensure the signal from nearby tracks does not interfere with the track being currently read<br>Both EP and SP mode are supported, showing how much more sensitive the EP mode is to alignment (see this post 0014 for more information)<br>VHS Simulator Quick Reference⌗<br>The VHS simulator will output the following indications, naming from top to bottom:<br>Graphic indication of the tape transport path. A schematic indication illustrating the alignment of the tape to the video drum, position of the A/C head. All distances exaggerated<br>Graphic indication of the A/C head alignment - azimuth, X-value offset, tilt<br>RF envelope - predicted RF envelope as seen after the head amplifier of the VCR<br>Hi-Fi RF envelope - predicted Hi-Fi RF envelope that accounts for narrower width of the Hi-Fi track<br>Head switch pulse - shows signal that selects which head is connected to the amplifier. See also &ldquo;head switch offset&rdquo; parameter<br>CTL pulses - simulation of the CTL pulses as read by the A/C head<br>Relative audio phases - 10x zoomed in view of the Hi-Fi, linear audio left and linear audio right waveforms. This indicates the relative phase between all of these signals, which will vary with X-value adjustments and A/C head azimuth<br>RF envelope CH1 - output of the CH1 video head, including the dead zone when it is not touching the tape<br>RF envelope CH2 - same for CH2 video head<br>Linear audio R - right channel of the linear audio track (shown at the same scale as the RF envelope)<br>Linear audio L - left channel of the linear audio track<br>Hi-Fi audio - audio picked up from the Hi-Fi system. The audio will vanish if Hi-Fi is predicted to not have been reliably established<br>Underneath there is a list of parameters for the model that can be edited and the tracking meter (I couldn&rsquo;t really fit it anywhere else). The tracking level meter is made to work the same way as it does on my BR-S800U - it shows the integrated RF envelope.<br>This is a short reference for all the model parameters, see below for a more detailed explanation of the physics involved:<br>ParameterDescriptionPlayback modeSelects between SP or EP playback mode (EP tape runs at 1/3rd of the rate the SP does, the tracks are much more narrow and the EP video heads are accordingly adjusted)Tape speedCurrent tape playback/shuttle speed. 1.0x means normal playback, 0.0x is still mode, -1.0x is reverse playback modeTracking knobTracking adjustment knob - allows adjusting tracking same way it works on my BR-S800U (offset by one SP track width - in EP mode it crosses over multiple tracks)Supply offsetVertical offset of the supply side tape guide pole. Adjusts how the tape enters the video drumTake-up offsetVertical offset of the take-up side tape guide pole. Adjusts how the tape leaves the video drumX-value adjustmentOffset along the direction of tape flow of the A/C head position that defines phase relationship between CTL pulses and video track positionsHead switch offsetAllows adjusting where the VCR performs a head switch. It&rsquo;s mostly just illustrative of the mechanic, it is usually supposed to be set to 0%A/C head tiltSee A/C head graphic for visual illustration of the tilt angle. Affects how well the tape comes in contact with CTL head and linear audio R/L heads. Very simplistic modelling just for illustrative purposesA/C head azimuthAzimuth of the A/C head. See graphic. Affects the phase relationship between CTL, linear audio R/L tracks.A/C head heightVertical height adjustment of the A/C head that models the vertical misalignment between the A/C individual heads and the tracks on the tapeCH1/CH2 azimuth errorAzimuth deflection of the video head (see &ldquo;azimuth scanning loss&rdquo; chapter for more...