![encoder stereo fm transmitter encoder stereo fm transmitter](https://electronics-diy.com/schematics/1139/Tube-Mosfet-Hybrid-Headphone-Amp.jpg)
The B channel is inverted and passed to the second additive mixer, together with the non-inverted A channel, to give the A-B signal. Both of these are passed to an additive mixer to form the A+B signal. The two stereo channels (A and B channels) are each passed through a buffer amplifier, where any filtering and pre-emphasis is added. The B channel (right) is now isolated and we have isolated our two A and B stereo signals. Inverting the (A-B) signal, then doing the same again, we get: In other words, the A channel (left) is now isolated. The stereo decoder demodulates the (A-B) signal and adds it to the (A+B). To decode this signal, then the audio range of frequencies from 20Hz to 16kHz form a mono signal (A+B). This is provided as a low-level 19kHz tone, that is doubled to 38kHz in the decoder. In order to decode the DSB 38kHz A-B signal, the receiver needs a carrier source. I have used the colour blue to identify the low channel audio frequencies (the ones you hear), and yellow for the high frequencies. The above diagram shows the composite stereo modulating signal. The 38kHz carrier is removed and the resultant sidebands are transmitted. The encoder then goes on to subtract the B channel from the A channel, and modulates a 38kHz tone, to generate a "difference-between-the-channels" signal. This is done by adding the left (A) channel to the right (B) channel, and transmitting this as an audio modulating signal, in the range of 20Hz to 16kHz. The basic principle of encoding stereo is to generate a modulating signal that can be received by both mono and stereo receivers alike. The result gave VERY nice stereo, probably because I had eliminated the "too many cooks" in the design. I was even able to misuse a spare switch in the CD4016 to eliminate the need for a flip-flop, as used in nearly every other design.
![encoder stereo fm transmitter encoder stereo fm transmitter](https://i.pinimg.com/originals/1a/c9/35/1ac935ddc7d8fdd002154d6db104205c.gif)
I then took it a stage further and simplified the filtering. I turned back to the tried and trusted CD4016 "chopper" circuit, then wondered if I could simplify it even further.
![encoder stereo fm transmitter encoder stereo fm transmitter](https://pira.cz/stk2pcbp.gif)
Time passed, and ran out, so I simplified the circuit. Have you ever tried building a circuit with the maximum number of components? I think I did! An Op-Amp also adds propogation delay, so I added more amps in the other signal paths to compensate - Sheesh!. Phase changes also vary with frequency every time one adds a new filter. When you add filters, you are also adding signal delay. I had thought about a very high quality circuit, but ended up with a multitude of problems. I began the design around Christmas 2004, with loads of workbench space occupied with rats-nests, and loads of operational Amplifiers, all glued to wooden boards, with their legs in the air. So I shall begin with a stand-alone stereo coder.
#ENCODER STEREO FM TRANSMITTER PDF#
If you want to have a look at this chip then you can find the datasheet on my PDF pages. There is a dedicated IC, the BA1404, that does exactly this, but the chip has stability problems, and several designs have suffered with the chip latching, not to mention that it is almost obsolete. The object was to produce a high-quality WBFM broadcast transmitter, with in-built stereo encoder. In my mind I designed and built several projects, including this one. I have not even been on the HF for months :-(Īlthough I may be out in the garden digging, or sitting in a hospital queue, the mind is still active. It seems that these days there is always something more sensible, important or very necessary, to consume all my spare time. In recent months I have been rather quite, when it comes to new projects.