6,032 research outputs found
Real-time speech encoding based on Code-Excited Linear Prediction (CELP)
This paper reports on the work proceeding with regard to the development of a real-time voice codec for the terrestrial and satellite mobile radio environments. The codec is based on a complexity reduced version of code-excited linear prediction (CELP). The codebook search complexity was reduced to only 0.5 million floating point operations per second (MFLOPS) while maintaining excellent speech quality. Novel methods to quantize the residual and the long and short term model filters are presented
Differential encoding techniques applied to speech signals
The increasing use of digital communication systems has
produced a continuous search for efficient methods of speech
encoding.
This thesis describes investigations of novel differential
encoding systems. Initially Linear First Order DPCM systems
employing a simple delayed encoding algorithm are examined.
The systems detect an overload condition in the encoder, and
through a simple algorithm reduce the overload noise at the
expense of some increase in the quantization (granular) noise.
The signal-to-noise ratio (snr) performance of such d codec has
1 to 2 dB's advantage compared to the First Order Linear DPCM
system.
In order to obtain a large improvement in snr the high
correlation between successive pitch periods as well as the
correlation between successive samples in the voiced speech
waveform is exploited. A system called "Pitch Synchronous
First Order DPCM" (PSFOD) has been developed. Here the difference
Sequence formed between the samples of the input sequence in the
current pitch period and the samples of the stored decoded
sequence from the previous pitch period are encoded. This
difference sequence has a smaller dynamic range than the original
input speech sequence enabling a quantizer with better resolution
to be used for the same transmission bit rate. The snr is increased
by 6 dB compared with the peak snr of a First Order DPCM codea.
A development of the PSFOD system called a Pitch Synchronous
Differential Predictive Encoding system (PSDPE) is next investigated.
The principle of its operation is to predict the next sample in
the voiced-speech waveform, and form the prediction error which
is then subtracted from the corresponding decoded prediction
error in the previous pitch period. The difference is then
encoded and transmitted. The improvement in snr is approximately
8 dB compared to an ADPCM codea, when the PSDPE system uses an
adaptive PCM encoder. The snr of the system increases further
when the efficiency of the predictors used improve. However,
the performance of a predictor in any differential system is
closely related to the quantizer used. The better the quantization
the more information is available to the predictor and the better
the prediction of the incoming speech samples. This leads
automatically to the investigation in techniques of efficient
quantization. A novel adaptive quantization technique called
Dynamic Ratio quantizer (DRQ) is then considered and its theory
presented. The quantizer uses an adaptive non-linear element
which transforms the input samples of any amplitude to samples
within a defined amplitude range. A fixed uniform quantizer
quantizes the transformed signal. The snr for this quantizer
is almost constant over a range of input power limited in practice
by the dynamia range of the adaptive non-linear element, and it
is 2 to 3 dB's better than the snr of a One Word Memory adaptive
quantizer.
Digital computer simulation techniques have been used widely
in the above investigations and provide the necessary experimental
flexibility. Their use is described in the text
Speaker Normalization Using Cortical Strip Maps: A Neural Model for Steady State vowel Categorization
Auditory signals of speech are speaker-dependent, but representations of language meaning are speaker-independent. The transformation from speaker-dependent to speaker-independent language representations enables speech to be learned and understood from different speakers. A neural model is presented that performs speaker normalization to generate a pitch-independent representation of speech sounds, while also preserving information about speaker identity. This speaker-invariant representation is categorized into unitized speech items, which input to sequential working memories whose distributed patterns can be categorized, or chunked, into syllable and word representations. The proposed model fits into an emerging model of auditory streaming and speech categorization. The auditory streaming and speaker normalization parts of the model both use multiple strip representations and asymmetric competitive circuits, thereby suggesting that these two circuits arose from similar neural designs. The normalized speech items are rapidly categorized and stably remembered by Adaptive Resonance Theory circuits. Simulations use synthesized steady-state vowels from the Peterson and Barney [J. Acoust. Soc. Am. 24, 175-184 (1952)] vowel database and achieve accuracy rates similar to those achieved by human listeners. These results are compared to behavioral data and other speaker normalization models.National Science Foundation (SBE-0354378); Office of Naval Research (N00014-01-1-0624
DeepVoCoder: A CNN model for compression and coding of narrow band speech
This paper proposes a convolutional neural network (CNN)-based encoder model to compress and code speech signal directly from raw input speech. Although the model can synthesize wideband speech by implicit bandwidth extension, narrowband is preferred for IP telephony and telecommunications purposes. The model takes time domain speech samples as inputs and encodes them using a cascade of convolutional filters in multiple layers, where pooling is applied after some layers to downsample the encoded speech by half. The final bottleneck layer of the CNN encoder provides an abstract and compact representation of the speech signal. In this paper, it is demonstrated that this compact representation is sufficient to reconstruct the original speech signal in high quality using the CNN decoder. This paper also discusses the theoretical background of why and how CNN may be used for end-to-end speech compression and coding. The complexity, delay, memory requirements, and bit rate versus quality are discussed in the experimental results.Web of Science7750897508
Synthetic speech detection and audio steganography in VoIP scenarios
The distinction between synthetic and human voice uses the techniques of the current biometric voice recognition systems, which prevent that a person’s voice, no matter if with good or bad intentions, can be confused with someone else’s. Steganography gives the possibility to hide in a file without a particular value (usually audio, video or image files) a hidden message in such a way as to not rise suspicion to any external observer. This article suggests two methods, applicable in a VoIP hypothetical scenario, which allow us to distinguish a synthetic speech from a human voice, and to insert within the Comfort Noise a text message generated in the pauses of a voice conversation. The first method takes up the studies already carried out for the Modulation Features related to the temporal analysis of the speech signals, while the second one proposes a technique that derives from the Direct Sequence Spread Spectrum, which consists in distributing the signal energy to hide on a wider band transmission.
Due to space limits, this paper is only an extended abstract. The full version will contain further details on our research
Time and frequency domain algorithms for speech coding
The promise of digital hardware economies (due to recent advances in
VLSI technology), has focussed much attention on more complex and sophisticated
speech coding algorithms which offer improved quality at relatively
low bit rates.
This thesis describes the results (obtained from computer simulations)
of research into various efficient (time and frequency domain) speech
encoders operating at a transmission bit rate of 16 Kbps.
In the time domain, Adaptive Differential Pulse Code Modulation (ADPCM)
systems employing both forward and backward adaptive prediction were
examined. A number of algorithms were proposed and evaluated, including
several variants of the Stochastic Approximation Predictor (SAP). A
Backward Block Adaptive (BBA) predictor was also developed and found to
outperform the conventional stochastic methods, even though its complexity
in terms of signal processing requirements is lower. A simplified
Adaptive Predictive Coder (APC) employing a single tap pitch predictor
considered next provided a slight improvement in performance over ADPCM,
but with rather greater complexity.
The ultimate test of any speech coding system is the perceptual performance
of the received speech. Recent research has indicated that this
may be enhanced by suitable control of the noise spectrum according to
the theory of auditory masking. Various noise shaping ADPCM
configurations were examined, and it was demonstrated that a proposed
pre-/post-filtering arrangement which exploits advantageously the
predictor-quantizer interaction, leads to the best subjective
performance in both forward and backward prediction systems.
Adaptive quantization is instrumental to the performance of ADPCM systems.
Both the forward adaptive quantizer (AQF) and the backward oneword
memory adaptation (AQJ) were examined. In addition, a novel method
of decreasing quantization noise in ADPCM-AQJ coders, which involves the
application of correction to the decoded speech samples, provided
reduced output noise across the spectrum, with considerable high frequency
noise suppression.
More powerful (and inevitably more complex) frequency domain speech
coders such as the Adaptive Transform Coder (ATC) and the Sub-band Coder
(SBC) offer good quality speech at 16 Kbps. To reduce complexity and
coding delay, whilst retaining the advantage of sub-band coding, a novel
transform based split-band coder (TSBC) was developed and found to compare
closely in performance with the SBC.
To prevent the heavy side information requirement associated with a
large number of bands in split-band coding schemes from impairing coding
accuracy, without forgoing the efficiency provided by adaptive bit
allocation, a method employing AQJs to code the sub-band signals together
with vector quantization of the bit allocation patterns was also
proposed.
Finally, 'pipeline' methods of bit allocation and step size estimation
(using the Fast Fourier Transform (FFT) on the input signal) were examined.
Such methods, although less accurate, are nevertheless useful in
limiting coding delay associated with SRC schemes employing Quadrature
Mirror Filters (QMF)
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