Re: Vdo Philips Car Radio Decoder Download

[Joao Charlesbois]

The idea is that you place this sensor outdoors, where it periodically sends temperature updates to the clock radio inside. I wanted to capture those transmissions and use the data programmatically, instead of just seeing it displayed on the clock radio. It occurred to me that a radio receiver ought to be able to pick up these transmissions.

vdo philips car radio decoder download

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I have several software-defined radio (SDR) USB dongles (such as this and this). It's pretty amazing what you can do with these low-cost devices, which generally have a range of around 25 MHz to 1.75 GHz. Beyond the obvious radio and TV broadcasts, I've picked up aircraft (ACARS) transmissions, weather data, pager and GSM traffic, and more. So I thought SDR might be a good way to see what this temperature sensor was sending to the clock radio.

I had no idea what frequency the temperature sensor used, but the ISM band seemed like a good place to start investigating, as many consumer-grade devices transmit around 433 MHz. The excellent and popular rtl_433 project supports a large number of 433 MHz devices, enabling you to decode protocols and output data in a number of formats (JSON, CSV, etc.).

The data packet was 36 bits long and repeated three times (with an extra starting nibble -- probably a sync signal of some sort -- on the first data row). rtl_433 couldn't decode anything from the data, which wasn't too surprising -- there are many, many types of sensors out there, each with their own proprietary format. If I wanted to make use of my sensor, I would have to reverse-engineer the protocol and write a decoder myself.

This showed a good, strong signal, with the long and short pulses clearly visible; the temperature sensor was using pulse-width modulation (PWM). With Audacity's selection tools, I could precisely determine the timings for the pulse and gap widths.

By manually changing the sensor's channel, placing it in the freezer, using an old battery, etc. I could see where and how the data packets changed. From there it was a relatively straightforward process to map out the protocol.

One final piece remained -- the last nibble in the packet changed based on the preceding data. I assumed it was a checksum or CRC of some sort. Basic checksum tests turned up nothing, so I turned to the excellent RevEng, which can calculate a huge number of CRCs for a given bit pattern. RevEng made short work of the data and determined it was using CRC-4 with a polynomial 0x9 and initial value of 0x1.

Having decoded the temperature sensor's protocol, it was easy enough to write up a decoder for it in rtl_433. A Github PR later, and rtl_433 now includes support for my Philips temperature sensor (see the code here).

The radio contains 9 valves and a number of semiconductors:

• ECC85 for FM rf amplifier and frequency changer,
• ECH81 as AM frequency changer and 1st FM IF,
• EF89 as AM/FM IF amplifier,
• EF183 as 3rd FM IF amplifier,
• ECC83 as pre-amplifier for left and right channels,
• 2x ECL86 as second AF amplifier and output amplifiers,
• EM84 tuning indicator and finally an
• EZ81 rectifier.
• AA119 germanium diodes as AM and FM detectors.
• 1 AF124 and 2 AF126 HF transistors and a number of AA119 diodes in the stereo decoder.
• An AC172 transistor serves as a pre-amplifier for the intercom function.
• The driver circuit for the stereo lamp, finally, uses 3 small-signal germanium transistors.

I got this radio in a barter for the B7X14A luxury plano. I wanted a plano with FM stereo that covered the full FM band. This one looked like it did not need much work, just some cleaning. Well... Something within the FM tuning mechanism was a bit stuck, so the needle only moved from 90-95 MHz. One of the metal skins of the pushbuttons was missing. And none of the pilot lights worked. Chassis taken out of case, seen from behind. Awkward to service.

I dusted and cleaned the inside and noticed the driver board for the stereo pilot light was dangling loose.The fixing screw had disappeared. After replacing the pilot lights, I noticed that the stereo light never lighted. Detail with stereo decoder in the back.

These Plano radio's have a diffusion screen to spread the light for the tuning scale illumination. This is a sheet of plastic suspended by 4 springs. As in many other planos, the plastic sheet had become brittle and cracked. It had fallen down and blocked the movement of the tuning mechanism. I replaced it by a sheet of professional drawing film. This is a mylar film with a matted coating on one side. It is slightly more transparent than the original film but does the job well. After this, I cleaned the case and the knobs thoroughly and treated the woorden parts with a liquid wax for fine wood. The radio is playing very well now. The sound is rather good, better than most of my other radios. This must be attributed to the large speakers. But it tends to distort when the volume is turned higher. And the bass is not really what it should be. This is probably caused by the extremely small ouput transformers. Except for this, I'm very satisfied with this last-generation valve radio. Copyright 2004 by Onno's E-page published 2004-07-03

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