Use the Spectrum 128 style menu below to navigate to the various sections detailing the SCART cable.

SCART Cable Main Menu Title
Introduction
Monitor Socket
SCART Socket
Lead Circuitry
Testing the Lead


Lead Circuitry

The simplest way to connect the Spectrum 128 to a SCART socket is to use the composite video (CVBS). This requires only the connections shown below. A 15 ohm resistor may be placed inline with the CVBS signal to reduce the 1.2V signal out from the Spectrum 128 down to the 1V level required by the SCART socket. For best results use screened cable and attach this to the connector shields at both ends, but make the connection prior to powering on the TV or Spectrum 128.

SCART Cable Composite Video Circuitry

Although the picture quality from the CVBS signal is significantly better than from the standard RF connection, it will usually suffer from colour fringing. To achieve the best picture quality requires connecting the Spectrum 128 to a SCART socket using RGB. However, this is more involved as it requires the three separate colour signals and a synchronisation signal.

The synchronisation signal may be obtained using the CSYNC line or the CVBS line. When using the CVBS line, it should be connected as shown above. Note that even though an RGB connection only requires synchronisation information to be delivered to SCART socket's VIDEO IN line, the CVBS signal can be used because a TV will generally ignore the picture information and just extract the synchronisation information required. It also has the advantage that the cable can be used in RGB or Composite Video mode. When using the CSYNC line, its voltage must be reduced using a resistor as shown below, and this results in a peak current of approximately 4mA.

SCART Cable Sync Circuitry

Each colour line must be reduced to a suitable level for the SCART socket and mixed with the BRIGHT signal to yield both colour shades. The circuitry required is shown below.

SCART Cable Colour Circuitry

The 150 ohm resistor reduces the colour signal down to the maximum level suitable for the SCART socket, and yields a peak current of 9.3mA. The current may be reduced by increasing the value of the resistor. This will darken the TV picture but may be compensated for by adjusting the brightness controls of the TV. The circuit operates as follows. When the BRIGHT signal is high, it has no effect on the colour signal level due to the diode and hence the bright shade of the colour is produced. For the normal intensity colour, the BRIGHT signal is low and the diode conducts thereby pulling down the signal level coming out of the Spectrum 128.

A drawback of the Spectrum 128 monitor socket is that, unlike the Spectrum +2A and +3, it does not provide a sound output. It is therefore necessary to obtain the sound signal from the MIC cassette port via a 3.5mm Jack plug. The SCART socket has separate connections for left and right audio channels and so these must be wired together. The voltage from the Spectrum 128 MIC socket is typically 200mV and this is directly suitable for driving the SCART socket's audio inputs. The SCART socket's audio ground can either be connected to the video grounds inside the SCART plug or to the ground signal from the MIC socket, but should not be connected to both as this might create a ground loop. The wiring of the audio signals is shown below.

Audio Circuitry

Two signalling voltages should ideally be presented into the SCART socket - BLANKING (pin 16) and FUNCTION SWITCHING (pin 8). The BLANKING input is used to instruct the TV whether an RGB signal or a Composite Video signal is being presented to the SCART socket, and the FUNCTION SWITCHING input is used to instruct the TV to automatically switch to display the signal being presented SCART socket. A voltage between 1V and 3V is required on the BLANKING input to select RGB mode, and voltage between 9.5V and 12V is required on the FUNCTION SWITCHING input to automatically select the SCART socket for viewing. The majority of problems experienced when attempting to connect a Spectrum 128 to a TV via SCART is the absence of correct voltages on these signalling inputs. If the TV allows manual selection of the SCART socket then it is not necessary to drive the FUNCTION SWITCHING input. Likewise, if the TV allows manual selection of RGB input mode then the BLANKING line can be left unconnected. However, most TVs only allow manual selection of the SCART socket but not which type of signal it is using. Unfortunately the default video type is Composite Video if the BLANKING input is left unconnected. Therefore, applying the correct voltage on the BLANKING input is far more important than for the FUNCTION SWITCHING input.

Unfortuantely, the Spectrum 128 monitor socket does not provide a power output that can be used to drive the BLANKING and FUNCTION SWITCHING inputs. There are a number of approaches for using other signal lines that have been widely suggested but these each come with limitations.

When the CVBS signal is used to the drive the VIDEO IN line, the CSYNC signal from the Spectrum 128 can be connected to the BLANKING input and will apply exactly 1V. However, some TV sets may be very harsh and deem this to be below the level needed to select the RGB input. A more significant drawback of this approach is that it heavily loads the CSYNC line and can cause instability of the Spectrum 128, with some people also suggesting it results in overheating of the ULA. This approach is therefore not recommended.

In theory it is possible to obtain power to drive the BLANKING and FUNCTION SWITCHING lines from either the KEYPAD socket, RS232 socket or the expansion bus. To use the RS232 socket or the KEYPAD socket would require an inline resistor of between 47 ohms to 620 ohms, and would yield a voltage range from 2.98V to 1.03V and hence a current range from 39.7mA to 1.67mA. It is better to opt towards the lower output voltage to minimise loading on the +12V line. The +12V could directly drive the FUNCTION SWITCHING line as its input impedance is at least 10k ohms, and so presents negligible loading. Note that since the KEYPAD and RS232 sockets deliver their +12V through a 180 ohm resistor, the voltage they apply to the FUNCTION SWITCHING input would be 11.7V, but this is still well within the required range. If the +12V is obtained from the expansion bus then the inline resistor required would need to be increased to between 220 ohms and 820 ohms, yielding BLANKING input voltages from 3.05V to 1.01V. The impedance that would be presented across the +12V from the KEYPAD socket, RS232 socket or the expansion bus would be fairly low and so other approaches may be preferable.

Using a voltage regulator can overcome the loading issue on the power line. Since the RS232 and KEYPAD sockets deliver their +12V line through a 180 ohm resistor, it is not ideal to use these as the input to the voltage regulator. So instead +12V could be taken from the expansion bus. The circuit below will generate 1.67V and so is suitable for driving the BLANKING input. The voltage may be increased to 2.1V by adding a second diode in series with the first. However, a SCART cable that must also plug into the expansion bus results in an inelegant solution, and so a neater approach is to simply to use an external source of power. This could be from batteries but it is preferable to use a regulated mains adapter.

SCART Cable Function Switching / Blanking Circuitry using Voltage Regulator

Putting all of the above theory together, the complete wiring of a RGB SCART cable suitable for the Spectrum 128 is shown below. For best results, use screened cable and attach this to the connector shields at both ends. Note that the design uses the CVBS signal such that the cable can also provide a Composite Video connection when an external +12V power supply is not attached. This could be replaced with the CSYNC line circuitry previous described if preferred.

Complete SCART Cable Circuit

Full circuit calculations can be found be here.