Sinclair QL

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The Sinclair QL (for Quantum Leap) is a personal computer launched by Sinclair Research in 1984, as an upper-end counterpart to the ZX Spectrum. The QL was aimed at the serious home user and professional and executive users markets from small to medium-sized businesses and higher educational establishments. It was a very promising and inventive machine, based on the 68008 processor from Motorola. Although it succeeded in becoming the first home computer based on a 32-bit CPU, the rush to market resulted in it being plagued by many problems from the start. After commercial disappointment, UK production was suspended in 1985 and finally discontinued in April 1986.

Models

There are two main versions of the QL are in circulation.

  • A certain number of boards to build standards up to Issue 5 were issued in either ROM or EPROM versions, sometimes referred to as pre-Issue 6.
  • The second, volume production, version of the board to build standard Issue 6, and subsequent, sometimes referred to as post-Issue 6.

There are also geographic variants.

  • European
  • North American

Video Information

The QL provides various video output capabilities. Two video modes are available:

  • 256×256 pixels (65,536 total) with 8 RGB colours and per-pixel flashing. Pixel aspect ratio was appx. 4.4:3 (i.e. 1.466) making the image extend into the horizontal overscan area of a TV. This has the designation 0.07M3.
  • 512×256 pixels (131,072 total) with four colours: black, red, green and white. Pixel aspect ratio was appx. 0.667. This supported 25 lines of 85 characters. This has the designation 0.13M3.

The supported colours can be stippled in 2×2 blocks to simulate up to 256 colours, an effect which does not copy reliably on a TV especially over an RF connection.

Video Circuitry

Internally, the TV picture generation section of the ZX8301 IC operates in conjunction with the memory mapped picture display area to produce five signals suitable for driving a colour monitor. These signals, red, green and blue (RGB), CSYNCL (composite sync) and VSYNCH (vertical sync) are routed to an 8-pin DIN female connector. Certain issues of the ZX8301 are very fragile, and even unplugging the RGB connection whilst in use has been known to damage the IC.

The ZX8301 generates the following:

  • Vertically: 28 blank lines, 256 image lines and 28 blank lines making for a total of 312 lines high. This can be seen on the OSSC as "312-p". It is not however really 312-p, which would require 555 pixels horizontally. Moreover only some PAL 576/2=288 lines i.e. not all 312 were displayed on TVs.
  • Horizontally: 640 pixels (64us) where the QL's 256 pixels needs 51.1us. This is slightly over the 48us where PAL guarantees visibility, which is why some 512-480=32 pixels might be missing when using a TV display.

On the official Sinclair Vision QL 12" colour monitor, the picture was made narrower so that all 512 pixels fitted the screen and also all visible lines.

Video Connectors

From the factory, the QL offers two video output connectors: RF and an 8-pin DIN connector with monochrome and colour composite output as well as the lines necessary for RGB.

A modification sometimes seen is a composite output tap cable, identified as a cable appearing from inside the computer with a coaxial cable terminated with a local connector e.g. in Belling-Lee connector in the UK.

SCART Cable

QL-to-SCART cables are available commercially. It is also possible to make a cable that works with your OSSC according to the following table. The QL pin numbers are per the QL User Guide / Concepts / Monitor diagram: be careful, there are multiple diagrams out there. Resistances are in series, and can be readily fitted inside the SCART connector. The resistances are necessary: the QL may be damaged if you have the RGB lines directly drive the low impedance (75 ohm) input expectations of the OSSC without the resistors. The SCART numbering assumes connection to a non-Japanese SCART (JP21), which used different connections.

OSSC-Compliant QL-to-SCART Cable
QL Name QL Level QL Pin In-Line SCART Pin SCART Level Impedance Notes
RED TTL Pin 6 680 Ω Pin 15 0.7 V 75 Ω Red
GREEN TTL Pin 8 680 Ω Pin 11 0.7 V 75 Ω Green
BLUE TTL Pin 7 680 Ω Pin 7 0.7 V 75 Ω Blue
VSYNC TTL Pin 4 10 Ω Pin 16 1-3 V 75 Ω Using blanking signal to indicate RGB selection
CSYNC /TTTL Pin 3 1.2 kΩ Pin 20 0.3 V sync 75 Ω Composite horizontal and vertical sync pulses provided via composite video input
GROUND - Pin 2 and shield - Pins 4, 5, 8, 13, 17, 18, 21 - - Various grounds

The 680 Ω resistors gets from TTL 5V to 0.7 Vpp. The 10 Ω resistor drops the 5 V to about 2 V, which is between the 1-3 V required to indicate RGB rather than composite 0-0.4 V. The 1.2 kΩ resistor drops the 5 V to about 0.3 V.

The OSSC does not require the SCART Status & Aspect Ratio up Pin 8 to be connected, unlike some older TVs which required 0 to 2 V → off, +5 to 8 V → on/16:9, +9.5 to 12 V → on/4:3.

The QL offers no readily accessible audio signal, so these connections simply ground the audio in the SCART connector.

OSSC-Specific Settings

The OSSC should be on AV1 (RGB-SCART) for the QL, as this input supports video in RGBS i.e. taking the RGB digital output from the QL and using its composite sync as a sync source.

OSSC-Experiments

Test equipment:

  • A pre-Issue 6 QL.
  • A RGB and sync cable as described above.
  • OSCC V1.6 board (i.e. HDMI) with version 0.88a firmware.
  • The input was identified as AV1_RGBS 312-p 15.62kHz 50.08Hz.

The target: the best fit should be double wide and triple high pixels to target a 1024x768 display.

Preliminary experiments were with LineMult mode. The OSCC defaults to mode 1. It would seem mode 3 results in no cropping and so is the best for now.

LineMult Experiments
LineMult IMode Info TV Resolution TV Ratio Notes
1 Preset 288p, IMode 312-p 50.08Hz, 539072/frame 720x288@50Hz 16:9 Edges and bottom exceed screen
Line2x Preset 288p, IMode 312-p 50.08Hz, 539073/frame 720x576@50Hz 4:3 Edges cropped, bottom exceeds screen
Line3x Preset 960x288, 312-p 50.08Hz, 539073/frame 1280x864x50Hz 4:3 Image visible, horizontally compressed
Line4x Preset 1280x288, IMode 312-p 50.08Hz, 539072/frame 1280x1152@50Hz 16:9 Edges exceed screen
Line5x Preset 1600x240L, IMode 312-p 50.08Hz, 539072/frame 1080p 16:9 Top and bottom exceed screen

Other Information

Inside the QL, the RGB and /CSYNC signals are also input to a RGB-to-PAL converter which produces composite PAL which was originally used to drive a domestic TV receiver. The same signals are mixed to produce a composite video signal to drive a standard monochrome monitor. When connected to a normally-adjusted TV or monitor, the video output would overscan horizontally. This was reputed to have been due to the timing constants in the ZX8301 chip being optimised for the flat-screen CRT display originally intended for the computer.

Further Reading

External Links

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