Analogue image techniques are used because they are much faster and less error prone than digital techniques. Image transmission in FSQ is very versatile, with two colour ima image sizes: higher and lower resolution, and a black-and-white very high resolution mode. The final image mode is a very high resolution black-and-white mode (like a FAX mode), ideal for sending documents. All modes are received automatically since they are managed using FSQ mode digital commands.
| Image Mode Summary | |||
| Mode | Resolution | TX Time | |
| LO RES COLOUR | 160 x 120 | 48 sec | |
| HI RES COLOUR | 320 x 240 | 192 sec | |
| B&W FAX | 640 x 480 | 256 sec | |
Thus there are three sizes of images. No matter what size the original image, or the mode used to transmit and receive it, when they are displayed, they are always shown (enlarged or reduced as necessary) at 640 x 480 pixels, in a floating window. Enlargement is achieved by interpolation, which gives excellent image appearance.
Actual size 320 x 240 pixel image.
The above image was received over the air under ideal conditions - image quality is remarkable given the modest resolution.
The modulation is similar to SSTV, i.e. it is analogue FM modulation, but that is where the similarity ends. The FSQ image modes use only 400 Hz bandwidth (similar to FSQ digital transmissions), and like FSQ, there is no synchronization transmitted. The ITU Emission Designator for the image mode is 400HF3C.
The start of the image is signalled digitally (i.e using FSQ), and the receiver simply captures data continuously for L lines of P pixels, resulting in a (L x P) pixel image, then reception stops. Each line in a colour image is sent three times, for red, green and blue components. The duration of transmission is (L x P)/400 seconds for colour, and (L x P)/1200 for B&W. The images are always sent in 4:3 landscape aspect ratio. Images are best sent and received in FSQCall mode, since then reception is automatic. Images can be sent to one recipient, to several recipients, or everyone on the channel, just as with other messages. Images can be sent and received in non-directed chat mode, but reception will need to be started manually.
Note: While transmitting or receiving images, it is most important that you refrain from typing in the TX buffer, moving windows around, realigning images and even looking for or opening other files. These interrupt-driven actions can incur delays, which are likely to disrupt transmission and reception. It's also not a good idea to do these things while transmitting text.
Narrow-Band Images
FSQ Image Mode was designed (like FSQ) for NVIS propagation on the lower bands, quite different from the intentions of SSTV. Unlike SSTV, the images are sent relatively slowly,
in a narrow bandwidth (⅓ of SSTV), and as a result the image signal
to noise ratio is generally better than SSTV for the same transmission power. As a guideline,
FSQ signals (before the image is started) need to be about +10dB SNR for noise-free image reception, although quite adequate
images may be received at or below 0dB if fading is limited.
FSQ images also offer significantly better reception than SSTV under NVIS conditions. Because there is no image sync, there is no risk of the image tearing due to timing changes, or breaking up during selective fades. All that happens is that the image may noise up momentarily, but will stay correctly aligned, as in the first example below.
Note that there is no hint of image break-up or tearing of the image in any of these examples, even with fading, and also no sign of multi-path ghosting. These images were all transmitted over a range of 300 km using a 15W transmitter.
2.
3.
Typical reception of FSQ 320 x 240 colour image mode
(1) 80m at night, with fades; (2) Live webcam shot of ZL1BPU, 80m at night (3) 40m daytime.
Image Format
The image mode sampling rate is 12,000 samples/sec, the same as the text mode. Each pixel consists of 10 samples, and there are 640/320/160 pixels/line. Therefore the line period is 0.5333/0.2667/0.1333 seconds, or 1.875/3.75/7.5 lines/sec. Each line is sent three times
(except in B&W FAX mode), in RGB order. Image transmission duration is given in the table above.
Images in portrait mode can be transmitted by rotating them before transmission, and rotating back after reception. Rotation is external to the FSQCall program (Windows Photo Viewer will rotate and save images). Original photographs of any size can be selected, and will be resampled automatically down to the appropriate size for transmission. A4 documents to be sent in FAX mode should be rotated to landscape format before transmission and returned to correct orientation again after reception.
No Sync
As mentioned, there is no horizontal (line) sync sent with the image, and no vertical sync during or at the start of the image. As a result,
it is possible for reception to start slightly earlier or later than the standard delay from the time of the command, depending on the relative speed of the computers used at either end. This has
two main effects: first, the image may be recorded shifted left or right, and the pixel order (in colour mode) may be wrong. This is
easily fixed after reception using the Phase control.
If the sound cards in the transmitting and receiving computers differ markedly in sample rate, the resulting image can be skewed. A timing difference of 24ppm is enough to cause a just noticeable (one pixel) slant. This is easily fixed after reception using the Slant control. High quality sound cards should be better than 5ppm, resulting in imperceptable slant, but some cheaper computer built-in sound modules and dongles may be rather worse.
