output signal

Input signal

Fig. 2

	clipping
	/ 1
	! К
1 . 1 t 1 1	r 1
	г 1 1

25

Such clipping of the output signal causes the generation of a number of unwanted harmonic waves in the amplifier. The overall effect of this phenomenon for the whole amplifier is known as the total harmonic distortion (THD). The percentage THD is obtained by comparing this distortion with the maximum amplifier output.

Unit 16

Task 6 The Digital Compact Cassette (DCC) (1992) was developed by Philips. Like

ordinary audio cassettes, this system uses magnetic tape but packaged in more robust form like a mini CD box. Unlike ordinary cassettes, digital recording is used with the result that the sound quality is as good as CDs - much better than ordinary audiotape. A major advantage over CDs is that the DCC can be re-recorded.

An attraction of DCCs is 'backwards compatibility'. This means that you can play conventional audio cassettes in the same machine as DCCs. There is no need then to throw away your cassette library. With a double-deck machine, you can re-record your cassettes in digital form. DCCs have several hours recording capacity.

Task 7

Describe this graph in sufficient detail for your partner to sketch it. If you have problems, the text which follows may help you

forward bias

Ip/mA 80-

60-

40-

20-0

0.6 0.8 V_rN

-100

germanium reverse bias

1„/цА

As a forward bias is applied to a germanium diode, the forward current remains negligible until a voltage of about 0.15V is reached. The forward current then begins to rise and, beyond 0.2V, it rises steeply. The forward current must be limited by resistance in the circuit to keep the diode within its power rating.

When a reverse voltage is applied, there is a very small reverse current (note the difference in the scale on the reverse axes). As the reverse voltage is increased to 10V, the reverse current rises steadily to 2uA. Between a reverse voltage of 10-100V, the reverse current is almost constant, only rising by about luA. Any further increase in reverse voltage causes the diode to break down and there is a very steep increase in reverse current. If not prevented, this will cause permanent damage to the diode.

Use this matrix to help you sketch your partner's graph.

1.2

1.0-

ш 0.8-

3

О

-о 0.6-

1_

CO

о 0.4-

reverse voltage (V) °-²"

—i 1—

0.2 0.1

0.1 0.2 0.3 0.4 0.5 0.6

-0.2 ш forward voltage (V) ф

-0.4 n> о

0.6 S>

0.8

Symptom

Cause

Remedy

Power doesn't turn on.

Mains lead is not connected.

Connect mains lead to the mains outlet.

Power is on but unit doesn't operate.

Safety devices are operating.

Turn off On/Off switch and disconnect mains cord. Then reconnect and switch on again.

TV programmes cannot be recorded.

Timer recording doesn't work.

Timer Record Function is set to OFF.

Set Timer Record Function to ON.

Clock is flashing 0:00.

Set clock time and perform timer setting.

Playback picture is not in colour.

Playback picture has large amounts of 'snow'.

TV set is not properly tuned to the video playback channel of the VCR.

Retune TV set.

Video heads are clogged ? with dirt.

Use new tape.

Task 4