Setting Up DSP Processors

1. Setting Up DSP Processors • Gordon Moore CTS • Gordon.moore@lectrosonics.com • 1-800-821-1121

2. DSP • Digital Signal Processors • Can be any device that modifies a digital signal (video, audio – anything) • In this class – we will be talking about Audio • NOT a “How-to” for specific manufacturers • Use their training

3. Apologies to any Manufacturers whose screens are not featured – no slight intended. The CONCEPTS are the core of this course - no brands endorsed or rejected.

4. What to expect Signal pathway organization Setting the array of “modules” or functions available

5. Why? Most DSP processors are barely used to their best capability. Units returned for service usually have just the simplest functions enabled while many enhancements are not touched.

6. Most commonly unused or forgotten Compressors Limiters Input filtering NOM bus Delays

7. Most commonly set up functions Input Gain (But often badly set) Routing (inputs to outputs) Equalization for outputs Controls interfaces

8. DSP organization • Flexible Architecture • Fixed Architecture • Dedicated function (one capability only) • Multi-function with fixed pathway Hybrid Architecture – some fixed, some flexibility in routing

9. Flexible Architecture All functions can be configured in a “drag-n-drop” environment. Audio functions can be placed almost anywhere along the signal chain in any order. Characterized by a drag and drop GUI (Graphical User Interface) and/or “fuel gauge”.

10. Sample

11. Advantages Complete flexibility – you can do some amazing things within a single box and develop very complex signal paths. Excellent choice for systems where complexity and/or multiple applications may come into play. Airports Large scale paging systems Complex communications systems Very little you cannot accomplish.

12. Disadvantages Flexible Architecture may be more costly on a per channel basis Requires more DSP power – memory register stacks must be allocated for any eventuality – code space cannot be optimized.

13. Fixed Architecture • Dedicated function • Does one type of function • Compression/limiting • Or • Equalization • Or • Signal Routing

14. Advantage Fixed architecture is simple to set up and operate – may not even require a computer Less cost for the box – may not be as cost effective as a combined DSP capability Sets up much like analog counterpart Excellent choice for existing system upgrade

15. Disadvantage Very limited in scope of function May not be very scalable

16. Fixed Architecture Multi-function Has multiple functions in a fixed pathway Generally fairly cost effective Limited in terms of routing and or set up choices. Advantage – predictable known good pathway No gauge Disadvantage – few if any routing choices

17. Hybrid Architecture • Combination of routing choices plus fixed multiple function signal pathway • Advantages – • Allows flexibility as far a signal routing goes – what inputs show up at what outputs • Optimizes DSP processing power – memory stacks and registers can be more tightly packed • Disadvantages • May not fulfill all needs in a system

18. Hybrid Architecture

20. Enough Boring stuff – Let’s set up some functions • Input Gain • Filters – Input and/or Output • Equalization • Feedback suppression • Crossovers • Noise reduction • Dynamics • Compressors • Limiters • Gates • Routing

21. Dynamics Those functions affecting gain structure and levels

22. INPUT GAIN • Most important setting – GET THIS RIGHT! • Always set up – but not necessarily well set up • Microphones • Handheld Vocals = 35dB minimum • Handheld Presentation = 45dB • Gooseneck desk = 45dB • Boundary mic = 55dB • Any further away = 60db+ • Ceiling – as hot as you can get it

23. INPUT GAIN • Multimedia • Unbalanced? Consumer = +10 • Balanced? Professional = 0 to -8

24. Signal to noise ratio Poor S/N Ratio Line Level Mic Level Noise Floor Mic Preamp Mixer Signal processors Amps

25. Signal to noise ratio Line Level Good S/N Ratio Mic Level Noise Floor Mic Preamp Mixer Signal processors Amps

26. Signal to noise ratio Line Level Mic Level Noise Floor Mic Preamp Mixer Signal processors Amps

27. THRESHOLDS • The level at which the desired function becomes active • Generally speaking a lower threshold level means it will activate earlier. • Recommended starting threshold for most line level (post preamp) functions = 0dBu

28. AGC and/or Levellers • Automatic Gain Control • RAISES gain if signal too low • Compresses if signal too high • BE VERY CAREFUL with these • Can run a room into feedback if used on amplified inputs • Primary application – to capture weak signals for recording or transmission • Start with threshold set at 0dB – keep gain centered at line level

29. Ambient Level Control Uses a reference microphone to measure room noise level that automatically adjusts system for noisier environment Reference microphone may be dedicated microphone – only purpose is reference, or may be designated microphone – used in system but designated to be the reference signal Read the manual

31. COMPRESSORS Control dynamics - loudest to softest Useful for keeping level under control Meek versus motivational speaker Rarely set up

32. Compressor settings - Ratio • Ratio – The amount of actual level increase above threshold that will yield ONE decibel in actual gain change after the compressor. • Example – 3:1 ratio • For every 3DB the gain increases above threshold, the final level will change only one dB • SO, if level jumps 9DB, the final level will jump only 3dB • FM is broadcast at typical 10:1 ratio

33. Compressor settings – Threshold, Attack, Release • Threshold - Level at which compressor begins to engage and affect level. • Attack – time in milliseconds the compressor begins to make changes after level exceeds threshold • Release – time in milliseconds the compressor lets go after level settles below threshold. • Makeup or post compressor gain • Compensation in level to make up for compressor reduction in signal.

34. Suggested setting for compressors • Speech systems (conference rooms, boardrooms, etc) Ratio = 3:1 Attack = 10-20ms Release = 200-500ms Threshold = 0 If initial input gain was set to result in 0dB level, then it would take a 60dB increase at the mic to hit the +20dB limit of input (clipping) MUSIC or Multimedia – try increasing ratio to 6:1 BE CAREFUL – too much = bad

35. LIMITERS • Basically, a compressor with an infinite ratio • Absolute ceiling to maximum level • Protects downstream gear by preventing severe clipping and overdriving amps and speakers • Many amplifiers have built in limiters to protect themselves. • ALWAYS set limiter threshold above threshold of compressor • Otherwise, compressor will never engage • Good for spikes like dropped microphones, cymbals, plosives (P,D,T)

36. Suggested Limiter settings • Threshold – 15db higher than compressor = +15dB if 0dB for compressor • Attack – faster than compressor = 2ms or quicker • Release – 200ms or less

37. Some dynamics filters can be frequency specific Compressor with low pass filter good for controlling proximity effect while allowing high frequencies to pass unaffected.

38. EXPANDERS Increases gain if signal very low such as weak talker BE VERY CAREFUL HERE – Expanders, in an amplified environment, can push system into ear bleeding feedback. Primarily intended for recording and or transmission.

39. GATES Gates activate a channel, allowing it to pass, once the level passes above the threshold. Found in some automixers Useful for noise control (noisy multimedia source, for example) Originated in music

40. Attack , Release, Threshold • Attack – try relatively fast settings, 1ms to begin • Release – start at 50ms • Threshold – depends on place in chain • If after input gain – 0dB is a good starting place • Lower if not getting a reliable start

41. Noise Gate

42. Spectral Affects the frequency response of the system

43. EQUALIZATION (Inputs) • Equalization is one of the most commonly used functions • Input EQ is generally for tonality control – adjusting the tonal content so each input sounds similar.

44. EQUALIZATION (Outputs) Generally used for speaker compensation Adjusting for “quirks” or characteristics in the loudspeaker response. You cannot EQ a “room”

45. Equalization patterns Pass – Low, High or Band Shelving – Low or High Parametric - Notch Graphic

46. “Q” No, not James Bond’s gadget guy The ratio of filter width to depth at 3dB roll off points

47. Low Q – Wide band width

48. High Q – Narrow Band width

49. Filter Slope or “Order” • Rate of attenuation on filter - Shown in terms of dB/octave • Octave – doubling of frequency • First order = 6dB per octave • Second Order = 12dB per Octave • Third Order = 18dB/Octave • Fourth Order – 24dB/Octave • Each order equals another 6db of roll off. • That means 4 times factor in power level • If 6 db down from 100watts = 25 watts.

50. First Order – 6dB per octave (High pass)