DSP based Speech Coders

1
DSP based Speech Coders VoN Systemsby Robert
Slaviero Analog Devices Australia

• IEEE Signal Processing Society Victorian
  Chapter
• Tuesday, 30 April, 2002
• IEAust Auditorium, Melbourne, Australia

2
Speech Coding (speech compression)

• Speech Coding is the process of digitizing speech
  at a reduced bit-rate in order to make more
  efficient use of transmission bandwidth or
  storage mediums
• Applications
• Voice based telecommunications over limited
  bandwidth transmission link (e.g.
  videoconferencing ISDN (H.320), PSTN (H.324),
  LAN (H.323), satellite, radio, etc)
• Voice storage forward systems (recording,
  processing, archiving)
• Voice over Packet Networks (VoN/VoP e.g. VoIP,
  VoDSL, VoATM, VoFR, VoCable
• Primarily real-time applications

3
Speech Coding (speech compression)
Transmission Link
ADC
Encoder
Decoder
DAC
Or
Microphone
Speaker
Storage Medium
4
Speech Coding Commercial Use Acceptance

• Technology advances (hardware, software, bit-rate
  / speech quality)
• Reducing costs
• International standards (e.g. ITU, ETSI, TIA,
  etc)
• Global interconnectivity / interoperability
  widespread use
• Applications evolution / development
• Telephony (PSTN) based 3.4kHz, 64kbit/s,
• circuit switched ? packet switched (e.g. IP)

5
Speech Coders - Technical Issues

• Bit-rate (commercial telephony use)
• Techniques
• Waveform coders (PCM based)
• Source / parametric coders - vocoders (LPC
  based)
• Hybrid coders (e.g. CELP, VSELP, RPE)
• Quality - toll or near-toll quality (MOS /
  perceptual, real-world, QoS)
• Delay
• Frame size, Algorithmic delay, Processing delay,
  Codec delay, Channel delay, System delay
• Other commonly used proprietary and
  international standards
• GSM, EVRC, VSELP, QCELP, MELP, MBE, etc

6
ITU Speech Coders - Technical Summary
7
DSP MIPS / Memory Channel Density

• DSPs evolution
• Became capable of executing speech coders
  (single-chip solution)
• Further increases in MIPS capability
• Further increases in internal memory
• Single-core DSPs ? multi-core DSPs
• General purpose DSPs ? application specific DSPs
• Now Multiple Instance (Multi-channel) capable
• Usually MIPS rather than memory limited
  (currently)
• Density improvements
• Either more channels of the same module /
  function
• or many different modules / functions (e.g.
  VoN, VoIP, etc)
• or both i.e. many channels of many functions
• Focus on high density VoN implementations /
  applications

8
Fixed-point DSP Implementation MIPS memory
(typical)
Figures based on ADSP-218x (except for G.4k),
PMProgram Memory (24-bit), DMData Memory
(16-bit), PM/DM Memory per extra channel
9
Typical VoN Components

10
Typical VoIP System Layout
11
Typical Product Applications
High Port Count (Infrastructure)
Media Gateway, VoDSL Gateway
Wireless Systems
Remote Access Concentrator or DSLAM

• Analog Devices
• VoN Solutions

IP PBX, MTU, or Enterprise Gateway
Voice/Data (SOHO/Residential)
Low Port Count (Premises)
IP telephony card
12
ADI DSP Channel Capacity Examples
13
MIPS Channel Density per DSP
G.726
VoN(G.711)
G.729A
G.723.1
VoN(G.729A)
G.728
VoN(G.723.1)
Assumes MIPS not memory constrained
14
Product Design Implementation Considerations

• Functionality / features
• Spec interpretation
• Target hardware options / selection
• Timescales
• Costs
• Development cycle / ease
• Tools, APIs, testing, optimisation
• Resource Usage (MIPS, memory (PM/DM/ext), MI)
• Channels per DSP (optimized code)
• Power consumption per channel
• Board space per channel

15
Trends / Goals

• Reduced bit-rates
• Improved speech quality
• Reduced complexity (MIPS)
• Reduced delay
• Adaptive coders
• Focus on implementation efficiency
• Increased channel / function density, lower
  power, less board space
• Resilience to environment transport medium
  impairments /
• corruptions (noise suppression, tandeming,
  delay, packet handling, etc)
•  

16
Contacting ADA
Analog Devices Australia previously
Signal Processing Associates - SPA
Multimedia Telecommunications DSP Software
Unit 3, 97 Lewis Road, Wantirna, Victoria,
3152, Australia Tel [phone] Fax 61 3
9800 2111 Email spa.info_at_analog.com Web
www.analog.com/spa Robert Slaviero Managing
Director robert.slaviero_at_analog.com