1、-_ CCIR VOLUME*X-1 90 4855212 0503995 5 118 Rec. 641 RECOMMENDATION 641 * DETERMINATION OF RADIO-FREQUENCY PROTECTION RATIOS FOR FREQUENCY-MODULATED SOUND BROADCASTING (Question 46/10, Study Programme 46L/10) (1986) The CCIR, CONSIDERING (a) Recommendation 638); (b) - the frequency separation betwee
2、n the wanted and unwanted carrier; - the maximum peak deviation; - the energy distribution both with spectrum and time of the modulation signal; - the pre- and de-emphasis characteristics: - the reception mode (monophonic or stereophonic); - the audio characteristics of the human ear (appropriately
3、taken account of by using the weighting network of Recommendation 468); . - the receiver input voltage; (c) of which are: - the selectivity characteristics; - - - the audio-frequency response, that the radio-frequency protection ratio is closely related to the audio-frequency protection ratio (see t
4、hat this relationship depends on a number of technical parameters, including: that the radio-frequency protection ratio further depends on the receiver characteristics, the most important the characteristics of the limiter and the stereo decoder; large signal performance and sensitivity; UNANIMOUSLY
5、 RECOMMENDS 1. that the objective two-signal method of measurement described in Annex I should be used for the determination of radio-frequency protection ratios in frequency-modulation sound broadcasting with transmission standards using a maximum frequency deviation of f 75 kHz and a pre-emphasis
6、of 50 p; 2. that the results obtained by this method should be checked by subjective listening tests whenever possible. ANNEX I OBJECTIVE TWO-SIGNAL METHOD OF MEASUREMENT FOR TRANSMISSION STANDARDS USING A MAXIMUM FREQUENCY DEVIATION OF f 75 kHz AND A PRE-EMPHASIS OF 50 PS 1. Measuring method The ob
7、jective method for the measurement of RF protection ratios is essentially a psophometric two-signal method in which the interfering transmitter is modulated by a standard coloured noise signal with a given frequency deviation. The interference effect is measured at the audio-frequency output of the
8、receiver by means of an international standard noise voltmeter (psophometer). The reference value used to define the audio-frequency signal-to-interference ratio is that which is measured at the audio-frequency output of the receiver with the same noise voltmeter, when the wanted transmitter is modu
9、lated with a sinusoidal tone of 500 Hz, while the interfering transmitter is switched off. * This Recommendation replaces Report 796 which is hereby cancelled. CCIR VOLUME*X-L 90 W 4855232 0503996 7 Rec. 641 119 2. Psophometer The noise voltmeter used to measure the wanted and interfering signals at
10、 the output of the receiver consists of a quasi-peak voltmeter with defined dynamic characteristics and an added filter which modifies the interfering frequencies according to their subjective interference effect (see Recommendation 468). As the noise voltmeter is also used to determine the referenc
11、e level and to adjust the frequency deviation, it must be possible to disconnect the weighting network. If, instead of the noise voltmeter, only a peak programme meter is available, it should have the same dynamic performance. 3. Noise signal for modulating the interfering signal generator The stand
12、ardized coloured noise is described in detail in Recommendation 559 (see Note). The spectrum beyond the required bandwidth of the standardized coloured noise should be restricted by a low-pass filter having a cut-off frequency of 15 kHz and a slope of 60 dB/octave. The audio-frequency amplitude/freq
13、uency characteristic of the modulation stage of the signal generator should not vary by more than 2 dB up to the cut-off frequency of the low-pass filter. Note. - Recommendation 571 proposes a different coloured noise signal. The effect which would result from the use of that signal instead of the o
14、ne proposed in Recommendation 559 is discussed in CCIR, 1978-821. 4. Measuring arrangement Figure 1 shows a schematic diagram of the measuring arrangement. The arrangement is suitable for monophonic and stereophonic transmissions. With stereophonic operation, channel A or B should be measured. The u
15、nwanted transmitter is always operated in the monophonic mode, because thk gives the more critical disturbing effect. A: B: C: D: E: F: G: H: J: C 500 HZ AF generator (for transmitter line-up procedure) calibrated AF attenuator noise generator noise shaping filter complying with Recommendation 559 c
16、alibrated AF attenuator 15 kHz low-pass filter pre-emphasis network stereo coder signal generator (wanted signal) FIGURE 1 - Diagram of the measuring apparatus K: L: M: N: O: P: Q: R: S: calibrated RF attenuator T: signal generator (unwanted signal) U: RF band-pass fdter (tuneable) frequency-meter f
17、or measuring the vi: frequency difference between signai generators J and L v3 : frequency-deviation meter coupling device v4 : matching network receiver under test calibrated RF attenuator Vi : 15 kHz low-pass filter psophometer (switchable weighting network) selector switch for the modulation sele
18、ctor switch. for modulating one or other of the signal generators selector switch for the frequency- deviation meter (J and L generators) selector switch for measuring the AF signai levels CCIR VOLUME*X-L 90 4855212 0503997 9 H 120 Rec. 641 It is important that the unwanted transmitter should be rel
19、atively free from spurious emissions. For this reason, a tunable bandpass filter with a 3 dB bandwidth of approximately 300 kHz, follows the signal generator. Care should be taken to avoid any interaction between the output stages of the two signal generators. A directional coupler may be used. The
20、audio-frequency signal-to-interference ratio should be measured at the audio-frequency output of the receiver ahead of the tone-control. If this is not possible, the tone-control should be in a position to assure a fiat amplitude/frequency response. The 19 kHz pilot-tone must be sufficiently suppres
21、sed at the audio-frequency output. It is necessary to add to the receiver output a low-pass filter with a 15 kHz cut-off frequency having an attenuation at 19 kHz of at least 40 dB. 5. Frequency deviation of the signal generators The accuracy of the measurement depends very much on the precision wit
22、h which frequency deviation of the signal generator can be set; this is especially true for the unwanted transmitter. The line-up procedure therefore should be carried out very carefully. For the determination of the reference level, the wanted transmitter J is frequency modulated with the aid of th
23、e tone-generator A, using a sinusoidal tone of 500 Hz. Attenuator B is then adjusted to obtain a deviation of f 75 kHz, including the pilot-tone in stereophonic operation. The reading of the noise-meter U, with the weighting network filter switched off, indicates the reference level. During the rema
24、ining measurements, the wanted transmitter is unmodulated. In stereophonic operation, only the pilot-tone has to be transmitted. The unwanted transmitter L is then modulated with a 500 Hz sinusoidal tone obtained from audio- frequency generator A. Attenuator B is then adjusted to obtain a deviation
25、of & 32 kHz (see Note). The audio-frequency level at the input of the unwanted transmitter before the pre-emphasis is now measured by means of the noise voltmeter U. The noise-weighting network is switched off. Next, a noise signal C + D replaces the sinusoidal tone, and attenuator E is adjusted to
26、obtain the same peak-reading as before at the noise voltmeter. The (quasi) peak-deviation is thus equal to f 32 kHz. Since the pre-emphasis has not been included in the level measurement, the actual peak deviation is higher. The described adjustment corresponds to the present-day broadcasting practi
27、ce. Note. - A normal sound-broadcasting programme without compression is simulated by modulating the unwanted transmitter with the standardized coloured noise signal using a frequency deviation of f 32 kHz. Therefore, the results obtained with this method and this deviation are only valid for sound
28、broadcasting programmes without compression. 6. Radio-frequency wanted-to-interfering signal ratio By means of attenuator K, the radio-frequency level of the wanted transmitter J is kept as low as possible to avoid non-linear effects in the receiver input stages. However, the level should be such th
29、at with the unwanted signal switched off, the audio-frequency signal-to-interference ratio is at least 56 dB. Attenuator N at the output of the unwanted transmitter is adjusted to obtain an audio-frequency signal-to-interference ratio of 50 dB at the audio-frequency output of the receiver S. In this
30、 case, the weighting network at the noise voltmeter U must be switched in. The ratio between the radio-frequency levels of the wanted and unwanted transmitters is the required radio-frequency wanted-to-interfering signal ratio, The measurement is carried out for channel spacings ranging from O to 40
31、0 kHz betwen the wanted and unwanted transmitters. The results should be given in a table or a diagram. If a diagram is drawn, the measuring points should be connected by straight lines. The radio-frequency level of the wanted transmitter and the receiver input impedance should be stated. - REFERENCES CCIR Documents 1978-821: 10/51 (Germany (Federal Republic of).
