speed sensitive volume control

I'm sure many of you have installed several head units that utilize the Speed sensitive volume control feature that increases the head units db as the vehicle speeds up in order to drown out road noise, as have I. However in most basic installs it is pre-wired in the factory harness (as its a factory feature). Now however I am doing a custom build from the ground up for a trike manufacturing company, and recently they want to utilze this feature in their trikes. So what exactly produces this signal that is supplied to the head unit for the speed sensitive volume control; Vehicle Speed Sensor, Tach signal, some sort of computer module...??? and how could I obtain this signal in a vehicle with no computer?

Thanks,

From what I have seen they either use an ABS sensor (which is 30+ pulses per 1 wheel revolution) or a sensor mounted on the transmission output shaft.

I, personally, have never seen a dedicated sensor for this feature, they basically just pick up a VSS signal that is already installed for other purposes (like ABS) and reuse it.

If I were going to add this to a vehicle I would definately go with the output shaft of the tranny.  You cand do a single pulse per revolution and it should work great.  You would then need a controller to turn the frequency that the output shaft is spinning at in to a something that the radio (or amp) can use to turn up the volume.

What kind of radio are you installing?

Currently we are using poineer premier, Sony, and JBL marine headunits, but I'm yet to recieve the brand specific info from the manufacturer, for this trike... I think it may depend on whether we can utilize this speed sensitive volume control or not. I am very familiar with the VSS shaft off of the tranny (We use 700R tranny's with a small block 350 setup) So I already have two types of pulse generators coming off of the shaft, one for the speedo (uses an inductive magneto to generate its own signal), and an auxilary VSS (that requires a 12V supply) to send signal to the Cruise control unit. So if I'm understanding this correctly... I should be able to either use a signal from one of my current generators, in which I'll need some sort of converter for the signal since the Speedo and Cruise for my specific setup require an 8000 pulse (per mile) generator setting? Or I'll need to add another auxilary VSS to the shaft dedicated solely to the radio, and still possibly need a converter for the signal to appropriate the frequiency dependent on which ever headunit we decide on? Also is there a technical name for this frequency converter, or will the audio manufacturer know what I'm talking about when obtaining the specs from them?

Thanks