Differences

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--- guides:ssdv [2012/09/13 19:58] – [Image Format] Adjust figures to reflect larger CRC fsphil
+++ guides:ssdv [2016/06/30 09:35] (current) – [Packet Format] fsphil
@@ Line 9: / Line 9: @@
 ^ Offset ^ Name ^ Size ^ Description ^
 |    0 | Sync Byte    |    1 | 0x55 - May be preceded by one or more sync bytes |
-|    1 | Packet Type  |    1 | 0x66 |
+|    1 | Packet Type  |    1 | 0x66 - Normal mode (224 byte packet + 32 byte FEC) |
+|      |              |      | 0x67 - No-FEC mode (256 byte packet) |
 |    2 | Callsign     |    4 | Base-40 encoded callsign. Up to 6 digits |
 |    6 | Image ID     |    1 | Normally beginning at 0 and incremented by 1 for each new image |
@@ Line 15: / Line 16: @@
 |    9 | Width        |    1 | Width of the image in MCU blocks (pixels / 16) 0 = Invalid |
 |   10 | Height       |    1 | Height of the image in MCU blocks (pixels / 16) 0 = Invalid |
-|   11 | Flags        |    1 | 000000xx: 000000 = Reserved, xx = Subsampling Mode (0 = 2x2, 1 = 1x2, 2 = 2x1, 3 = 1x1) |
+|   11 | Flags        |    1 | 00qqqexx: 00 = Reserved, qqq = JPEG quality level (0-7 XOR 4), e = EOI flag (1 = Last Packet), xx = Subsampling Mode (0 = 2x2, 1 = 1x2, 2 = 2x1, 3 = 1x1) |
 |   12 | MCU offset   |    1 | Offset in bytes to the beginning of the first MCU block in the payload, or 0xFF if none present |
 |   13 | MCU index    |    2 | The number of the MCU pointed to by the offset above (big endian), or 0xFFFF if none present |
-|   15 | Payload      |  205 | Payload data |
+|   15 | Payload      |  205 / 237 | Payload data |
-|  220 | Checksum     |    4 | 32-bit CRC |
+|  220 / 252 | Checksum     |    4 | 32-bit CRC |
-|  224 | FEC          |   32 | Reed-Solomon forward error correction data. This may be omitted for storage, or if the mode already provides FEC |
+|  224 | FEC          |   32 | Reed-Solomon forward error correction data. Normal mode only (0x66) |
-Sizes and offsets are in bytes, with each packet being 256 bytes in total (or 224 bytes with no FEC data).
+Sizes and offsets are in bytes, with each packet being 256 bytes in total.
 ==== Image Format ====
@@ Line 57: / Line 58: @@
 The current implementation uses 8-bit RTTY at various baud rates. A more appropriate method should be used in future.
+One idea is to pack the SSDV packets (sans-FEC) into an AX.25 frame for transmission over standard amateur radio packet networks. It may even be possible to do this in such a way that the format is compatible with the APRS network.
 ===== Software =====
@@ Line 65: / Line 67: @@
 http://ukhas.org.uk/projects:dl-fldigi
+NOTE: Versions prior to DL3.1 contained an older version of the SSDV decoder which can not handle the finalised format described above.
 ==== ssdv ====
@@ Line 87: / Line 90: @@
 ===== History =====
+  * 2015-10-19: An EOI flag was added to the header, using one of the previously reserved bits. This bit should be set to 0 except on the final packet for the current image.
   * Some final changes made to the format 2012-03-04. Images produced with this version of the encoder will be compatible with all future decoders.