1、Rec. ITU-R F.1112-1 1RECOMMENDATION ITU-R F.1112-1*DIGITIZED SPEECH TRANSMISSIONS FOR SYSTEMSOPERATING BELOW ABOUT 30 MHz(Question ITU-R 164/9)(1994-1995)Rec. ITU-R F.1112-1The ITU Radiocommunication Assembly,consideringa) that voice communications in the HF band use 3 kHz channels;b) that security
2、is essential for some communications;c) that scrambling is the only means of obtaining a sufficient level of security;d) that the required level of security can easily be achieved using digitized speech technology;e) that there is therefore a need for speech signal coders (vocoders) associated with
3、HF modems;f) that, for good quality HF channels, the maximum permissible bit rate is 4 800 bit/s;g) that interference and propagation effects such as fading cause an increase in the bit error ratio for digitalcommunications, thereby calling for correction processes (error-correcting codes, interleav
4、ing),recommends1 that, for short-range communications (using ground waves), vocoders at 2 400 bit/s or 4 800 bit/s should beused;2 that, for long-range communications (using sky waves), either vocoders at 2 400 bit/s or vocoders at 600-800or 1 200 bit/s with error-correction coding should be used, a
5、ccording to the quality of the link;3 that the digital radiotelephone systems used should comply with the general specifications set out in Annex 1and the specific specifications contained in Annex 2, 3 or 4, according to the type of vocoder involved.ANNEX 1General outline of HF digital radiotelepho
6、ne systemsAn HF digital radiotelephone system comprises a conventional radiotelephone circuit, a speech signal coder (vocoder),an optional scrambling facility and an HF modem. Figure 1 represents the block diagram of such a system.The transmitted speech signal is input to the voice coder, where it i
7、s analysed and converted into a bit stream. This bitstream, possibly after scrambling, is then applied to the modem, in which it is shaped for transmission in the telephonechannel frequency band. The bit stream from the modem receiver output, if necessary after descrambling, is applied tothe voice d
8、ecoder, where the speech signal is restored._*This Recommendation should be brought to the attention of Radiocommunication Study Group 8.2 Rec. ITU-R F.1112-1For high error ratios with which vocoders at low bit rates (2 400 or 4 800 bit/s) cannot cope, vocoders at a very low bitrate (600-800 or 1 20
9、0 bit/s) with error-correction coding should be used.Diversity reception can also be employed, by means of two receivers using space-diversity antennas, with subsequentprocessing in the receiving part of the modem.There are many types of vocoder at bit rates 4 800 bit/s, in particular channel vocode
10、rs, orthogonal vocoders and linearpredictive vocoders. Systems using those types of vocoder are described in Annexes 2, 3 and 4.FIGURE 1Block diagram of an HF digital radiotelephoneSpeechsignalSpeechsignalVoicecoderVoicedecoderVocoderInterfaceScrambler/descrambler(Scramblingoption)TransmitterReceive
11、rHF modemHF transmitterHF receiver 1HF receiver 2D01(Diversityoption)FIGURE 1/F.1112.D01 = 8.5 CMThe three types of vocoder all have a wanted bit rate of 2 400 bit/s and a sound intelligibility greater than 90% undergood transmission conditions. They are thus more or less equivalent in terms of qual
12、ity at that bit rate.All the vocoders are presented with the associated modem which in all three cases is of the “parallel modem” type(several sub-carriers independently modulated in the audio band).For the channel vocoder and the orthogonal vocoder, the short period of the elementary symbol transmi
13、tted by themodem (13.3 ms and 8.33 ms, respectively) make them less resistant to severe HF propagation conditions, characterizedby multipaths with a delay of a few milliseconds, which will cause significant inter-symbol interference.For the LPC 10 vocoder, when the associated modem is the one descri
14、bed by STANAG 4197, the symbol periodis 22.5 ms. This modem is thus less affected under such conditions.Furthermore, in the LPC 10 vocoder certain important frame bits are protected by a powerful correction code (Golaycode), with the result that the deterioration in performances as the bit-error rat
15、io (BER) increases is more gradual thanwith the previous vocoders: sound intelligibility falls to 80% for a BER of 2%, as against 1% for the other vocoders.Satisfactory operation also has to be guaranteed in poor conditions. This can easily be achieved with the LPC 10 vocoderby using simultaneously:
16、 data compression to reduce the wanted bit rate to 600/800/1 200 bit/s (at the cost of a drop in the vocoders intrinsicquality); insertion of redundancy, which increases resistance to errors.With this arrangement, in the 800 bit/s version, the threshold of 80% sound intelligibility is not reached un
17、til the BERexceeds 5%.Rec. ITU-R F.1112-1 3ANNEX 2Digital radiotelephone system using a channel vocoder1 System descriptionThis system is composed of a conventional HF radiotelephone circuit and a digital voice terminal.The speech input from a microphone is analysed and digitized by the channel voco
18、der. The digital signal is then appliedthrough the interface to the scrambler where it is scrambled with randomizing signals. The scrambler output goesthrough the interface to the modulator and is converted to a voice-frequency signal by the FDM-D-QPSK method.The audio signal output of the radio rec
19、eiver is demodulated into digital codes by the FDM-D-QPSK demodulator. Thedigital code signal goes through the interface to the descrambler where it is translated into the original digital codes.These signals go through the interface to the channel vocoder to restore the speech signal, which is appl
20、ied to atelephone receiver.2 Channel vocoder2.1 TheoryThe channel vocoder divides the speech-band signal of about 300 to 3 000 Hz into a number of contiguous spectralbands, and measures the strength of each band. These measurements are coded and transmitted.Figure 2 shows the channel vocoder theoret
21、ical block diagram. In the analyser section, a number of bandpass filters(BPF; filter-equivalent processing) are used to separate the speech bandwidth and to pick up the frequency component ofeach spectral band. The BPF outputs are measured to determine the level of each band. At the same time, “voi
22、ced” and“unvoiced” sounds and pitch frequency are detected. These parameters are sampled and quantified at the analyser, whichformats a 2 400 bit/s coded signal for transmission.f1f2fnf1f2fnFIGURE 2Block diagram of channel vocoderSpeechinputVoiced/unvoicedsound detectorPitch detectorCoderDecoder2 40
23、0 bit/sDigitaldata(Analyses)AdderSwitchNoise generatorPulse generatorVoiced/unvoiced soundPitch(Synthesizer)D02SpeechsynthesizedsignalFIGURE 2/F.1112.D02 = 2 CM4 Rec. ITU-R F.1112-1In the synthesizer section, a noise generator and an impulse generator excite the spectrum synthesizer, the noisegenera
24、tor for unvoiced sounds and the impulse generator for voiced sounds. The output frequency of the impulsegenerator is set almost equal to the pitch frequency. The output of the noise generator or the impulse generator isanalysed by a BPF configuration similar to the analyser section. The levels of an
25、alysed spectral bands are multiplied andadded to recover the speech signal.2.2 ImplementationThe digital voice terminal uses FFT (Fast Fourier Transform) for speech frequency spectrum analysis. The calculatedspectrum is separated into a number of spectral bands whose bandwidth is equivalent to that
26、of the BPF. The spectralbands or channels are averaged to determine the level of each spectral band. The frequency is detected by obtaining themaximum auto-correlation, while voicing-unvoicing detection is based on the level of the maximum values of thefunction. On the other hand, speech is synthesi
27、zed by generating the impulse response of the BPFs with an FIR (FiniteImpulse Response) digital filter. The result is multiplied by the output level of each spectral band. Finally, thewaveforms of all spectral bands are added to get a speech signal.3 FDM-D-QPSK modemThe FDM-D-QPSK modem applied in t
28、his system is basically the same as described in Recommendation ITU-R F.763(see Annex 1). The major characteristics are: data rate is 2 400 bit/s, 18 tones are used, 16 of these with a spacing of110 Hz in the band 935-2 585 Hz being modulated in D-QPSK mode at 75 Bd signalling rate. A 605 Hz tone is
29、 used forthe correction of end-to-end frequency errors. An 825 Hz tone is chosen for synchronization to avoid excessive loss atthe band edge. Guard times between frames are introduced to combat multipath propagation and group-delay distortion.4 Test resultsFigure 3 shows the characteristics of a cha
30、nnel vocoder in terms of sentence intelligibility, sound intelligibility andsyllable intelligibility vs. BER. Figures 4 and 5 show the characteristics of FDM-D-QPSK modem in terms of BER vs.Eb/N0. Static characteristics are in Fig. 4 and characteristics with fading in Fig. 5.Rec. ITU-R F.1112-1 5506
31、070809010010 310 210 12 5 2 5FIGURE 3Characteristics of a channel vocoderBERPercentage(%)Sentence intelligibilitySound intelligibilitySyllable intelligibilityD03FIGURE 3/F.1112.D03 = 10.5 CM51015 210 310 41010 5252525FIGURE 4Static characteristics of FDM-D-QPSK modemBiterror ratioE /N (dB)b 0D04E :N
32、 :b0energy per bitnoise power spectrum densityFIGURE 4/F.1112.D04 = 14 CM6 Rec. ITU-R F.1112-115 20 25 30 35 40 45 41010 110 310 2252525FIGURE 5Characteristics of FDM-D-QPSK modem with fadingD/R = 49D/R = Bit errorratio0E /N (dB)bDoppler frequency: 0.5 HzTime delay: 1 ms D/R: r.m.s power ratio betwe
33、en direct wave and reflected waveD05FIGURE 5/F.1112.D05 = 13.5 CMANNEX 3Digital radiotelephone system using anorthogonal vocoder1 System outlineFigure 1 represents a block diagram of a type of digital radiotelephone system. The circuit includes the conventionalradio equipments of a main HF link: tra
34、nsmitters, receivers for dual space-diversity reception, antennas and trunkcircuits.The line terminal equipment consists of a vocoder for conversion of the speech signal into a 2 400 or 4 800 bit/s bitstream and a modem to communicate with the HF radio.Rec. ITU-R F.1112-1 72 VocoderOperating tests w
35、ere carried out on the digital radiotelephone circuit using two types of vocoder designed for operationat 2 400 and 4 800 bit/s. The block diagrams of the vocoders, both orthogonal, are shown in Figs. 6 and 7. The initialspeech signal is applied to the spectrum analyser, which determines the values
36、Ykof the speech signal spectrum envelopeat different frequencies.The 4 800 bit/s vocoder takes 30 samples evenly distributed within each of three sections of the total speech signalfrequency range. These samples are converted into 16 coefficients jof spectrum envelope decomposition into anorthogonal
37、 series, which with a 60 Hz frame frequency constitute a 3 840 bit/s binary sequence. Simultaneously with thespectrum analysis, the value of the fundamental frequency period is extracted from the speech signal as well as the“fundamental frequency-noise” excitation characteristic, which are transmitt
38、ed at a double frame frequency of 120 Hz inan 8-digit code, thus occupying 960 bit/s of the total bit stream.At reception, the signals jwhich control the orthogonal spectrum synthesizer, and the fundamental frequency andfundamental frequency-noise signals, which control the synthesizer excitation ge
39、nerator, are extracted from the receivedbit stream. The synthesizer excitation generator generates either a pulse group sequence at fundamental frequency andpossessing a uniform spectrum, or a pseudo-random pulse sequence. The speech signal is synthesized at the synthesizeroutput.The 2 400 bit/s voc
40、oder (see Fig. 7) is orthogonal with non-linear conversion, in which the orthogonal series is thesquare root of the speech spectrum envelope. The samples Ykof the spectrum envelope number 21 here, the distancebetween them increasing smoothly with frequency following the curve of equal articulations.
41、 These samples aresubjected to square root extraction Yk, after which ten coefficients jof square root decomposition from the spectrumenvelope into the orthogonal series are determined. The values of these coefficients are transmitted every 20 ms in thetotal bit stream by 4-digit code combinations,
42、also at 20 ms intervals.At reception, the 2 400 bit/s stream is split up into the component signals, among which the fundamental frequency andfundamental frequency-noise signals control the synthesizer excitation generator, which is analogous to that in the4 800 bit/s vocoder, and, in parallel, two
43、spectrum synthesizers. The output of the first synthesizer is connected up to theexcitation output of the second synthesizer, which both squares the spectrum to be synthesized and establishes a linearrelation between the initial and the synthesized speech spectra.312y1y311631216y2y3A B C D EF QFIGUR
44、E 6Block diagram of a 4 800 bit/s orthogonal vocoderSpeechsignalSpeech signalFundamental frequencyFundamental frequency-noise4 800 bit/sDigitalchannelFundamental frequencyFundamentalfrequency-noiseD06A:B:C:D:spectrum analyserorthogonal transformertransmission interfacereception interfaceE:F:Q:synthe
45、sizerfundamental frequency extractorexcitation generatorFIGURE 6/F.1112.D06 = 10 CM8Rec. ITU-R F.1112-1ABCDEQFEyy1220y1y2y2012101210FIGURE 7Block diagram of a 2 400 bit/s orthogonal vocoderSpeech signalFundamental frequencyFundamental frequency-noise2 400 bit/sDigitalchannelFundamentalfrequency-nois
46、eFundamentalfrequencySpeechsignalA:B:C:D:spectrum analyserorthogonal transformertransmission interfacereception interfaceE:F:Q:synthesizerfundamental frequency extractorexcitation generatorD07FIGURE 7/F.1112.D07 = 21 CM PLEINE PAGERec. ITU-R F.1112-1 93 ModemThe modem is a 2-PSK multi-channel device
47、 using orthogonal signals. Its basic technical characteristics are as follows: sub-channel rate: 100 or 120 bit/s, number of channels: 20, channel frequency separation: 142 Hz, orthogonality interval: 1/142 s, length of protection interval: 1.29 or 2.29 ms, channel signal reception method: optimum n
48、on-coherent.The modem is designed for the following rates: channel rate 4 800 bit/s: information rate 4 800 bit/s, channel rate 4 800 bit/s: information rate 2 400 bit/s (information doubling on sub-channel pairs with maximumfrequency spacing), channel rate 2 400 bit/s: information rate 2 400 bit/s
49、(1-PSK).Furthermore, under 4 800 bit/s rate, the switchover to the 2 400 bit/s information rate is effected by Halley code.The modem can also be used for reception from two space-diversity antennas.4 Tests4.1 MethodThe operation of the digital radiotelephone circuit was studied over a period of several months at different times of yearon latitude and meridian paths ranging from 1 500-3 000 km, and up to 10 000 km with repeaters.Two methods were used for quality assessment: measurement of the intelligibility of the
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