Fall, 2012Slow-Scan TV over FM radio using software defined radioSlow scan television is a method for transmitting still images. It is used mainly by amateur radio enthusiasts. While the old analog broadcast TV (NTSC) requires 6MHz bandwidth to transmit video and audio, SSTV uses a much narrow bandwidth of about 3KHz. Of course the transmission rates are much lower and take a few seconds to minutes to transmit. Even though we are in the internet age, SSTV is still very active among amateurs. As an anecdote, SSTV was used to send images from space in the early days. In fact, the international space station astronauts still occasionally send images on amateur radio frequencies (145.8Mhz) using the SSTV protocol. You can visit http://www.issfanclub.com/, the ISS fan club webpage to see if the SSTV is active and when is the ISS going to be visible in your area. The task of this lab is to use the wide-band FM (WBFM) transmitter and the SDR you received and design a transceiver system for SSTV. SSTV uses narrowband analog frequency modulation (NBFM) to transmit images. Brightness in the image is encoded as frequencies in the audio range (1500-2300Hz). Color images are achieved by transmitting green-blue-red signals sequentially. There are many modes of transmission for SSTV. Here we are going to use Martin M1, which is one of the most popular schemes to transmit SSTV. It is mostly popular in Europe. Because it is relatively straight forward we will use it here. With Martin M1 it takes 114 second to transmit a 256x320 pixel image. We will transmit the Martin M1 signal over broadcast FM radio using the WBFM transmitter, receive it on another computer using the SDR, demodulate it, and decode it to produce images. There are several level of complications in the project which I will partly base the grade on. The minimum requirement to get a pass grade is to be able to demodulate the WBFM signal. The second is to be able to transmit a valid image coded with Martin M1. The third is an SSTV transceiver system. The fourth is “free-style” make-your-own modification to the Martin M1 method to achieve higher frame-rates. Beyond that, you can add many other layers of processing which I will mention towards the end of the document You can work in pairs. The programming platform is up to you. Most students will probably would want to use Matlab, but other languages like python, java, C, and C are possible (and preferable if you are up for it!). Deliverables:
Evaluation criteria: (not by order of importance)
Specifications:WBFM demodulation:Your FM transmitter modulates an audio input using frequency modulation. It's range is within US broadcast FM radio band 88.1-107.9Mhz. You can use the FM transmitter frequency at intervals of 100KHz, but I recommend using odd frequencies to minimize chances of interference with broadcast FM stations. The broadcast FM band is split into 200KHz slots. This is relatively a large bandwidth and therefore it is also called wideband FM as opposed to narrowband FM which can be as low as 5 Khz. The bandwidth of each channel is about 150KHz, though sidebands do leak outside of this range. In FM radio the information is encoded by modulating the frequency of the carrier, Here, is the carrier frequency, is the frequency deviation and is a normalized baseband signal. The broadcast FM baseband signal, , consists of mono (Left+Right) Channels from 30Hz to 15 KHz, a pilot signal at KHz, amplitude modulated Stereo (Left - Right) channels around KHz. (See http://en.wikipedia.org/wiki/FM_broadcasting for more information). The baseband signal is: This is the spectrum of :
Pre-emphasis and de-emphais: Both the mono and stereo signals are pre-emphasized using a single-pole high pass filter before modulation. This is because noise in FM modulation is larger for higher frequencies. By emphasizing the high frequency signal, we can improve the overall signal to noise ratio. It is likely that the your FM transmitter performs pre-emphasis. Because of the pre-emphasis, we must compensate in the receiver by filtering the demodulated signal with a single-pole low-pass filter with a cutoff frequency of 2122Hz. Now, the SDR already demodulates by the carrier frequency , so the samples you get are actually where is the sampling rate. The demodulation can be done in the same way we did in the labs, or with other methods, such as: http://www.embedded.com/design/configurable-systems/4212086/DSP-Tricks--Frequency-demodulation-algorithms- or, http://www.chrec.org/pubs/MILCOM09_B1.pdf
Tips:
SSTV:VIS codeAll standard SSTV modes utilize a unique digital code that identifies the mode to a receiving system. The code is called the VIS, or Vertical Interval Signal code. Although the entire calibration header is often referred to as the “VIS code”, the code itself is only a part of it. The seven-bit code is transmitted least-significant- bit (LSB) first, and uses “even” parity. The code for Martin M1 is 44d (decimal). The VIS packet is transmitted before the image information:
The VIS packet consists of 300ms Leader tone at 1900Hz, 30ms break tone at 1200Hz, another Leader tone at 1900Hz, 30ms VIS start bit at 1200Hz, Seven 30ms bits where 1100Hz = “1” and 1300Hz = “0”, 30ms of parity bit where Even=1300Hz and Odd = 1100Hz, 30ms VIS stop bit at 1200Hz. Martin M1 Mode:The Martin M1 Mode is pretty straight-forward. It is based on FM modulation and encodes the intensity of the color components in frequency. The color mode is RGB with frequency range of 1500-2300Hz to in code the luminance. The line scan has 320 pixels and there are 256 lines. The scan sequence in each line consists of Green, Blue and then Red. Below is a diagram of Martin M1:
Each line starts with a ms sync pulse at 1200Hz. It is followed by a ms separator at 1500Hz, a (0.4576ms/pixel) Green scan at 1500-2300Hz, a ms separator at 1500Hz, a Blue scan at 1500-2300Hz, a ms separator at 1500Hz, a Red scan at 1500-2300Hz and a final ms separator at 1500Hz. The total transmission is therefore 114.3 seconds. Specs and Tips On modulator:
Specs and Tips On Decoder:
Transmit and Receive Applications Once you have debugged all the above and you are convinced they work, you will need to develop two simple applications. These will provide a user interface and display for transmitting and receiving images. Specs and Tips on the Transmitter
Specs and Tips On Receiver
General Tips
Useful Links:
Beyond…..Faster SSTV? There are many possible extensions:
please let me know if there are any problems. -- Miki |