File size for Digital SSTV
The HDSSTV DOS programs by Barry Sanderson KB9VAK are used in DIGTRX and DIGISSTV. HDSSTV encodes a file to be transmitted into a Windows wave file (.WAV). The file's data bytes are placed into blocks of a fixed size that contain the file data and redundant bytes. Higher levels of redundancy requires more blocks to be encoded. Each block contains 306 symbols. In the first block, 17 symbols may be used for the filename and overhead. The file's data bytes and redundant bytes make up the rest of the symbols in each block. The last block may be filled with as many of the filler symbols as necessary to full up the block. The total number of blocks determines the transmission time. Thus, there are set increments to the transmission time based on file size and the level of redundancy.
Minimum & Maximum file size
The minimum file size is a no longer a problem. The bug in the DOS routines has been corrected.
The maximum file size is limited in DIGTRX by the size limit on the capture buffer of 10,002 KB or 465 seconds (7 minutes and 45 seconds). At 10% redundancy, 50KB is the largest file that can be encoded and decoded by DIGTRX.
Below is an example of a 50,000 byte file received with DIGTRX.
![[bf48050k.png]](bf48050k.png)
The image above is a screen capture showing the reception of a 640x480 JPEG 2000 image that was 50,000 bytes. The transmission time was 467 seconds including the "two tones" at the beginning of the transmission. The received wave file did not include these tones so 464 seconds of audio was recorded. The encoding required 183 blocks. The lowest redundancy, 10%, was used. These paramaters represent the maximum limit for file size received with DIGTRX.
Update : The latest version of DIGTRX now allows for up to 10 minutes of audio recording. This should allow for files as large as 64,000 bytes to be received if encoded at 10% redundancy.
File size and transmission time
It is not likely that you would want to make a file for transmission that would take nearly eight minutes to send, but you do need to keep in mind the size limitations and the consequences of large files.
File size and transmission time chart
size.txt
To download on some browsers, just hold down the "Shift" key before clicking the link.
Select the file size for the desired transmission time
IrfanView allows the user to pre-select a target file size when saving in JPEG 2000 (JP2). After selecting JP2 as the file format to save, click the Options button and select "Compress to" and fill in the box for the number of bytes. Using the chart above, select a byte number 8 or 10 bytes less than the file size for the transmission time desired. This will alow the best quality picture for the time of transmission desired.
Reducing picture file size
If your picture files are too large, or the quality is too poor, you may need to do some image editing. First crop the image to remove unnecessary background. Resize or resample the image to a lower resolution. Generally most users are satisfied with dimentions of 320x240. That is 320 pixels wide and 240 pixels tall. Always maintain the original aspect ratio. Reducing the number of colors before saving may save some bytes in the saved file. Next is saving the image with compression. The content of the image and the save type (format) will greatly affect the saved image quality. A black and white image may be color reduced to two colors and saved as a GIF file for greater efficiency. Line drawings do better when saved in GIF or PNG. Busy photos with lots of detail do best in standard JPEG (JPG). Photos with lots of soft areas or portions out-of-focus, do best saved in JPEG 2000 (JP2). Using the right combination of image editing and compression can yield beautiful images that will fill the screen and have a reasonable transmission time.
Dave Jones KB4YZ