The issue you describe is most likely caused by non-synchronized video signals between the Macintosh and the external display device.
If your external display device is not configured to receive VSYNC at the rate at which the Macintosh is transmitting then rolling lines will result. *You might notice that the "roll" is proportional to the degree at which the monitor's sync and the computer's sync are disharmonized.
It is also possible that the pins of the Macintosh video output and the display device's connector do not correspond, causing the display to not receive the signal (sync) pulses altogether.
In either case it is necessary to attach an adapter between the Macintosh video port and the display cable to adjust the frequency and/or align the pins. There are several companies who offer adapter solutions, including Black Box Corp., Enhance Cable Technology, Griffin Technology and MacAdapt.
- For more information about a specific third party search the TIL for the vendor's name.
- For a detailed explanation of audio and video technology see Murray Wheeler's "Multimedia Glossary", TIL article #16536, 16357, and 16538.
Below is a brief description of a few terms commonly used when discussing video sync issues.
Horizontal Sync
A signal created and used to synchronize the horizontal scan of a video signal, often combined with vertical sync into a composite sync. This signal is derived by dividing sub-carrier by 227.5 and then doing some pulse shaping. The signal is used by monitors and cameras to determine the start of each horizontal line.
Refresh Rate
A term used to describe the number of times the screen is completely scanned every second. For example, if a monitor has refresh rate of 60 hertz, then the entire monitor screen is scanned 60 times per second.
Sync
Short for synchronous. Refers to signals used to synchronize the horizontal and vertical scans of a video signal. This signal is derived from a composite or combination of horizontal and vertical drives, with some slightly narrowed and delayed pulses as well as the addition of equalizing pulses. When used, is usually accompanied by subcarrier.
Synchronous
Able to perform two or more processes at the same time, such as sending and receiving data, by means of a mutual timing signal or clock.
TTL
Abbreviation for "transistor-transistor-logic", a bipolar integrated-circuit logic that uses transistors with multiple emitters. TTL is a saturated form of logic; during turn-on both the emitter base and collector-base junctions are forward biased, causing an accumulation of charged carriers in the base region. As the device is turned off it is discharged through the collector. The time required for this discharge results in a delay in turning the transistor off. This is referred to as storage-time delay. *Usually the term "TTL" is used in the context of "TTL signal", which basically means a type of waveform.
Signal-to-noise Ratio (S/N)
An S/N ratio can be given for the luminance signal, chrominance signal, and audio signal. The S/N ratio is the ratio of noise to actual total signal, and it shows how much higher the signal level is than the level of noise. It is expressed in decibels (dB), and the bigger the value is, the more crisp and clear the picture and sound will be during playback.
Vertical Sync
A pulse used to trigger the vertical retrace of a scanning electron gun from the bottom of a frame back to the top.
Vertical Sync Pulse
A portion of the vertical blanking interval which is made up of blanking level and six pulses (92% duty cycle at -40 IRE units) at twice the horizontal sync pulse repetition rate. Synchronizes vertical scan of television receiver to composite video signal. Starts each frame at same vertical position (sequential fields are offset 1/2 line to achieve interlaced scan.