8
The sync separator U19, which is a vertical sync output, is used to trigger a re-
triggerable one shot. The one shot timing is adjusted by the current into C22 from
the current source comprised of Q1 and U38. The current source is adjustable
with R28 over a range such that timing can be varied from 100ms to 1 second.
This has the effect of delaying the absence of video for up to one second. If
vertical pulses become present before the one second time-out the video
presence will not indicate a loss of video. This keeps the logic from switching to
the opposite input for extremely short losses of video.
Audio Path:
Audio Path:
Audio Path:
Audio Path:
Only one path will be described since all four inputs and paths are alike. The
input comes into a differential stage U39A, with high impedance buffers to keep
the input impedance at or above 100K ohms. U1 provides the actual differencing
of the balanced inputs. Common mode signals such as AC line hum are rejected
by this stage. This stage has a gain of –6 dB which is made up in the balanced
output stage. The output of U1 feeds the switch of U9 which selects either A
inputs or B inputs. The output of the switch feeds an adjustable gain stage U11,
which compensates for gain inaccuracies of the output stages. The output is a
balanced output which has a gain of +6 dB when measured across the output
positive to negative output pins.
As described above in the video path, form C relays are included in all four
output channels which provide a path from module input directly to module
output in the event of power failure of the ARTS-201B. All four channels of A
audio are also routed to NC contacts on relays K5, K6, K7 and K8. The NO
energized side of these relays comes from the four output amplifiers, U11, U12,
U13 and U14 and the common pin connects to the audio output connectors.
Should there be a power failure within the unit, all four relays drop to the relaxed
mode, and the audio input signal is routed directly to PGM output. When power is
restored, the relay coils are re-energized and the output signal is again derived
from the 2x1 switches U9 and U10. The relay coils are powered from the -15 volt
regulator U37.
