SSTV Scottie 1 Encoder explanation


Slow-scan television (SSTV) is a method for picture transmission used by amateur radio operators to transmit and receive images. SSTV uses analog frequency modulation, in which every different value of brightness in the image gets a different audio frequency. In other words, the signal frequency shifts up or down to designate brighter or darker pixels, respectively. Color is achieved by sending the brightness of each color component separately.

This post is a simple guide of how to generate SSTV Scottie 1. This encoder will be a part of our future project in RadioClub EIT consisting of an SSTV Encoder with an Arduino, to transmit pictures from the sky by radio.

Sample model

Scottie 1 sample output:

For creating this encoder, first we need to read the specification of this codification. Next lines will be a simple summary from barberdsp.com (Dayton SSTV forum, 20 May 2000).


SCOTTIE 1 MODE

Properties

PropertyValue
VIS CODE60d (0111100)
COLOR MODERGB(1500-2300Hz)
SCAN SEQUENCEGren, Blue, Red
Number of lines256
Picture dimensions320x256 px
Transmission time109.6 s
Color scan time per line138.2440 ms
Color scan time per pixel0.4320 ms

Timing sequence

#DescTime[ms]Frequency[Hz]
1“Starting” sync pulse (first line only!)9.01200
2Separator pulse1.51500
3Green scan
4Separator pulse1.51500
5Blue scan
6Sync pulse9.01200
7Sync porch1.51500
8Red scan

Scottie 1 Calibration header with VIS code

IdentityFrequency[Hz]Time[ms]
Leader tone1900300
Break120010
Leader tone1900300
VIS start bit120030
VIS CODE Bit 0 - 1100Hz = “1”, 1300Hz = “0”130030
VIS CODE Bit 1130030
VIS CODE Bit 2110030
VIS CODE Bit 3110030
VIS CODE Bit 4110030
VIS CODE Bit 5110030
VIS CODE Bit 6130030
Parity - Even = 1300Hz, Odd = 1100Hz130030
VIS stop bit120030

Simple encoder example

This code is a part of an Arduino Project. It’s very ilustrative of how Scottie 1 SSTV is generated.

In this example, myFile its an stream in which are stored in binary the pixels of the picture in the form RedByteGreenByteBlueByte. Each pixel are encoded with a 24-bit RGB.


//Methods prototypes
void transmit(int freq, int duration);  // Generate a sinus signal with the given freq and time
void myFile.read(); // Return the next byte of a stream (myFile)

/** VOX TONE (OPTIONAL) **/
transmit(1900, 100);
transmit(1500, 100);
transmit(1900, 100);
transmit(1500, 100);
transmit(2300, 100);
transmit(1500, 100);
transmit(2300, 100);
transmit(1500, 100);

/** CALIBRATION HEADER **/

transmit(1900, 300);
transmit(1200, 10);
transmit(1900, 300);
transmit(1200, 30);
transmit(1300, 30);    // 0
transmit(1300, 30);    // 0
transmit(1100, 30);    // 1
transmit(1100, 30);    // 1
transmit(1100, 30);    // 1
transmit(1100, 30);    // 1
transmit(1300, 30);    // 0
transmit(1300, 30);    // Even parity
transmit(1200, 30);    // VIS stop bit

/** STARTING SYNC PULSO (FIRST LINE ONLY)  **/

transmit(1200, 9);

/** TRANSMIT EACH LINE **/
while(myFile.available()){

  // Read line and store color values in the buffer
  for(uint16_t i = 0; i < 320; i++){
    buffR[i] = myFile.read(); // Read Next Byte
    buffG[i] = myFile.read();
    buffB[i] = myFile.read();
  }

  // Separator pulse
  transmit(1500, 1.5);

  // Green Scan
  for(uint16_t i = 0; i < 320; i++){
    transmit(1500 + 3.1372549 * buffG[i], 0.4320);    // .4320ms/pixel
  }

  // Separator Pulse
  transmit(1500, 1.5);

  // Blue Scan
  for(uint16_t i = 0; i < 320; i++){
    transmit(1500 + 3.1372549 * buffB[i], 0.4320);    // .4320ms/pixel
  }

  // Sync Pulse
  transmit(1200, 9);

  // Sync porch
  transmit(1500, 1.5);

  // Red Scan
  for(uint16_t i = 0; i < 320; i++){
    transmit(1500 + 3.1372549 * buffR[i], 0.4320);    // .4320ms/pixel
  }
}

That’s all, hope you enjoy this old but not least codification.

acien101@debian:~$ EXIT

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