Digital radio receiver with program-controlled mixing oscillator frequency

A novel receiver for subcarriers in a radio broadcasting system is described, in which the characteristic frequency of the mixing oscillator is varied under program control. An incoming signal from antenna 1 passes through an input stage 2 to a mixing stage 3. Mixing stage 3 receives the output of an oscillator 4. The output of the mixing stage passes into an intermediate frequency filter stage 5, which preferably includes A/D converters 13 and demultiplexers 14. Stage 5 feeds a plurality of demodulators 6, which may include equalizers 12. Buffer memories 7 store information from demodulated subcarrier signals, and feed an evaluation unit 8. Evaluation unit 8 has a first output which drives a speaker 9 and a second output which is applied to a memory 11. A control unit 10, connected to the output of memory 11, controls the frequency generated by mixing oscillator 4. This system facilitates digital signal transmission by compensating for poor transmission conditions which would otherwise cause serious dropouts in the digital signal.

DETAILED DESCRIPTION 
FIG. 1 illustrates a novel receiver for radio transmissions, which is 
suitable as a car radio, which is connected to an antenna 1 dimensioned 
suitably for receiving a carrier of a transmitter sending above the VHF 
band. The output of the input stage 2 of the car radio leads to a mixing 
stage 3, which is connected to a mixing oscillator 4. A suitable 
oscillator 4 is model no. SP 2002 from Plessey. The input stage may be of 
the kind typically used in television receivers. An intermediate frequency 
filter 5 is connected to the output of the mixing stage 3. A suitable 
filter 5 is model SFE 10.7 MA5 from Murata. This intermediate frequency 
filter 5 has a plurality of outputs, to which demodulators 6 for the 
subcarriers contained in the intermediate frequency signal are connected. 
Each of the demodulators 6 is tuned to a different multiplex subcarrier 
assigned to it. 
The signals demodulated by the subcarriers in the demodulator 6 are 
temporarily stored in read/write memories 7. A suitable memory is model TC 
55 465 from Toshiba. Connected to this group of buffer memories 7 is an 
evaluation unit 8, which has an output 8.sup.1 at which the signal for the 
loudspeaker 9 can be picked up. Suitable evaluation units include the 
Viterbi decoder model SQR 5053 from SOREP, the model PCM 55 from 
Burr-Brown, or an audio decoder as described in German Published 
Application DE-OS 34 40 613, THEILE. 
The characteristic frequency of the mixing oscillator 4 is variable by 
means of a control circuit 10, which may be, for example, the integrated 
circuit SN 74 LS 161. The characteristic frequency undergoes compulsory 
variation at certain time intervals, or in other words cyclically. The 
standard for the change in the characteristic frequency from one cycle to 
another is contained in a memory 11. A suitable memory is EPROM model 2716 
from NEC. 
In the exemplary embodiment described here, memory 11 has an input that is 
connected to a further output 8.sup.2 of the evaluation unit 8. The 
magnitude of the frequency change to be made upon the transition to the 
next cycle, if it is contained in the transmitted signal, can be picked up 
at this output. 
However, the predetermined frequency differences may also be stored in the 
memory 11 in the form of a table and called up from it by the control 
circuit 10. 
This type of receiver is part of a transmission system that compensates for 
the consequences of reduced reception capability as follows: The point of 
departure is a digital representation of an analog microphone signal. As a 
general rule, the sampling rate of the microphone signal is selected to be 
at least twice as high as the highest frequency to be transmitted. At the 
same time, the number of binary digits used to represent the instantaneous 
value can be selected to be no higher than the number that, multiplied by 
the duration of one bit, can be transmitted between two successive 
sampling instants. 
As a rule, however, higher sampling rates and shorter bit durations than 
required by the above conditions are selected. The duration of one bit 
must, on the one hand, not be selected to be so short that, in serial 
transmission, the differences, amounting to up to 100 microseconds, in 
transit time in the later multipath reception between the signals arriving 
at the antenna over the various paths would substantially impair 
recognition of the binary signals; in other words, the duration of one bit 
must suitably be long, compared with the time during which not all the 
"multipaths" are at the same level (high or low) and thus are still 
transmitting different values to the antenna. 
By selecting a plurality A of subcarriers, A bits can now be transmitted in 
parallel. The available transmission time for each bit is thus greater by 
a factor of A than in serial bit transmission. If the number is 
sufficiently high, then supplemental information, needed for controlling 
the receiver status, for instance for synchronizing purposes, can be 
inserted between each two sampling values as well. 
By varying the characteristic frequency of the mixing oscillator, the 
receiver can be switched to a different frequency without having to change 
the intermediate frequency filter or the evaluation circuit. This change 
in frequency, which naturally must be effected synchronously in both the 
transmitter and receiver, has the effect of shifting the transmission of a 
signal to a different frequency range for a certain period of time. This 
is advantageous if a certain frequency range exhibits major interference 
because of multipath reception conditions at the receiving location, at a 
time when other frequency ranges are exhibiting less interference. The 
receiver of the exemplary embodiment is particularly flexible to 
manipulate, because it can infer the magnitude of the next frequency jump 
or hop from the received signal itself. 
A change in reception conditions occurs especially often in a receiver 
installed in a moving vehicle. This means that the receiver according to 
the invention is particularly suitable as a car radio. 
The cycling times, in other words how long the receiver remains at a 
particular frequency, and thus how long the transmitter remains at that 
frequency, must be selected to be long enough that the evaluation circuit 
8, and the equalizers that may possibly be provided, can respond, and long 
enough that interference from the additional modulation of the subcarriers 
arising from the switchover will remain slight, compared with modulation 
by the bits. Experience has shown that no fewer than 100 bits should be 
transmitted during one cycle. For modulation of the subcarriers, the known 
methods of PSK (phase shift keying), DPSK (differential phase shift 
keying), QPSK (quadrature phase shift keying), FSK (frequency shift 
keying) or MSK (minimum shift keying), among others, are suitable. See the 
Schwartz text, cited above. If the various echo transit times over the 
"multipaths" are very long, it may be suitable to provide equalizer 
circuits 12 at the inputs to the demodulators 6. A suitable equalizer 12 
is model LCC 44, and a suitable demodulator 6 is described in the Proakis 
text pages cited at the beginning of the specification. These equalizers 
can then be adjusted by means of training sequences that are inserted into 
the transmitted signal. 
Suitably, the intermediate frequency filter 5 is completed with an A/D 
converter 13 and a frequency demultiplexer 14, so that splitting of the 
intermediate frequency block into the various subcarrier ranges already 
occurs on the digital level. A suitable A/D converter is model no. HS 10 
68 C from Sipex. Preferably, demultiplexer 14 includes a 
Finite-Impulse-Response (FIR) digital filter for each demultiplexed 
subchannel. 
In a broadcast radio system with a plurality of programs, the various 
programs to be broadcast can exchange channels with one another, in 
synchronism, upon the change of frequency. As a result, an overall 
increase in the requisite receiver bandwidth in practical operation is 
unnecessary. 
Various changes and modifications may be made, and features described in 
connection with any one of the embodiments may be used with any of the 
others, within the scope of the inventive concept. Suitable carrier 
frequencies used in Germany are as follows (subchannels spaced in 
increments of 0.0512 Hz=2.sup.9 .times.10.sup.-4): 
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FREQUENCY BLOCK NO. MEGAHERTZ 
______________________________________ 
1 221.0 
221.0512 
221.1024 
221.1536 
221.2048 
221.2560 
221.3072 
2 222.0 
222.0512 
222.1024 
222.1536 
222.2048 
222.2560 
222.3072 
3 223.0 
223.0512 
223.1024 
223.1536 
223.2048 
223.2560 
223.3072 
4 224.0 
224.0512 
224.1024 
224.1536 
224.2048 
224.2560 
224.3072 
5 225.0 
225.0512 
225.1024 
225.1536 
225.2048 
225.2560 
225.3072 
6 226.0 
226.0512 
226.1024 
226.1536 
226.2048 
226.2560 
226.3072 
7 227.0 
227.0512 
227.1024 
227.1536 
227.2048 
227.2560 
227.3072 
8 228.0 
228.0512 
228.1024 
228.1536 
228.2048 
228.2560 
228.3072 
______________________________________