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Digital Audio Systems – DESC9115 -‐ Final Written Review DrumMaster – DSP Product Proposal Created by Meisha Stevens June 2015
The DrumMaster will make your snares snappier, your kicks punchier and your high hats cleaner.
All without loosing the natural sound of your kit. -‐ This is the drum mic spill solution audio engineers have been waiting for -‐
1. Problem Description Multi-‐track drum recording is common practice in recording studios, however the vast amount of sound, varying timbres, frequency spectra and sound envelopes of each element of the kit makes it difficult to isolate and treat these elements individually. The problem many audio engineers face is dealing with leakage or spill in close mics, picking up unwanted sound from neighbouring sound sources for example, high hats audible in the snare close mic. The audio effects an engineer may wish to do to a snare drum, could be vastly different to the effects they want on the high hats, and therefore, the presence of leakage will ultimately affect how the high hat sounds in the overall mix, as a result of the processed snare containing high hat leakage. (Elias Kokkinis, 2013) “Leakage complicates audio editing, processing and mixing and it is a well known problem in drum recordings.” (Elias Kokkinis, 2013) Digital signal processors such as noise gates can remove the leakage, and isolate sound events making them more manageable for editing and mixing. Robjohns echoes this notion in his Sound On Sound workshop ‘Recording Drums’ stating, “The more spill, the less control, and the harder it is to balance the kit”. So why not just use a gate to solve this issue? Robjohns goes on to explain “Gates inherently chop off the opening transient of the wanted sound, changing the character of the sound, so the result starts to become more artificial and processed.” (Robjohns, 2003). Maintaining a natural sounding element to the drum mix is particularly important in typically ‘live’ sounding genres of music, such as rock, jazz, and country. The over-‐processed unnatural drum sound is less of an issue for other genres like pop, hip-‐hop and electronic music, and in fact, is often employed as an effect. The examples we will look at are for the broad umbrella of rock music, and will look at gaining control of each focal drum element while maintaining a natural feel, avoiding the artificial sound often associated with simple drum gating. The results of the DrumMaster include snappier sounding snares, punchier sounding kicks, clean defined high hats and isolated round sounding toms. All of these results can be achieved without loosing the natural feel of the drum kit, and without sounding over processed and choppy as is often the case with traditional drum gating alone. 2. Specification The primary function behind this effect is a developed noise gate that is called using a single multi-‐track drum mic recording as it’s input argument. Prior to the gating, the input signal is processed using a 3-‐band equalizer. The resulting output signal is then summed with the original unprocessed signal at a nominated mix ratio. Figure 1 demonstrates the signal flow in a block diagram. Figure 1. DrumMaster DSP -‐ Block Diagram.
The script for the DrumMaster calls 3 different developed functions, the first to normalise the signal (normalizsig.m) so that the maximum and minimum peak values are contained within the parameters of 1 and -‐1. Next is the equalisation function (EQ.m) which uses 3 separate sets of EQ, each with the capability to boost or cut specific frequencies over a given band width. Finally the Noise Gate (noisegtabs.m) function, which uses a set of parameters to allow sound to pass at a given upper threshold, and no sound to pass at a lower threshold, these thresholds are activated and deactivated at user defined rates called attack and release and will remain open for a nominated hold time. These working functions are included with the script package, along with audio examples for trial purposes. The digital signal processor has been kept modular, so maximum user control is available. The modules of the processing system can be expressed in the following five step mathematical sequence. 1. X1= x * n Normalise the input x 2. X2 = X1 * (EQ1, EQ2, EQ3) The normalised signal is process by 3 separate EQs 3. X3 = X2 * G The EQed signal is gated using the noise gate function 4. S = X3 * X1 The EQed and gated signal is summed with the normalised signal X1 5. Y = S * n The output Y is a result of the normalised Summed signal S x = audio input signal G = Noise Gate function EQ1, EQ2, EQ3 = 3 independent Equalisers X1, X2, X3 = 3 steps of signal processing n = Normalisation function Y = DrumMaster Output result S = Sum of 2 processed input signals X3 (normalised, EQed and gated) and X1 (normalised input) Different drum inputs will call for different parameter settings, however pre-‐sets have been developed and trialled, creating a suite of presets for an entire drum kit including the Kick, Snare, High Hats, Tom-‐toms, and Overheads. Figure 2 demonstrates how a skin for the digital signal processor may look as a plug-‐in for a DAS. The presets along the top will pre-‐determine the parameter settings below, however the user still has the option of controlling certain aspects, as not all drums will have the same characteristics as those used in the trial phase. Figure 2. DrumMaster plug-‐in graphic user interface prototype.
3. Implementation There are a number of user-‐defined parameters that can be selected in a preset or adjusted by the individual when using the DrumMaster. The first step is to select an input signal to be processed by the DrumMaster. A good starting point for a snare mic input would be to select the SnareMaster preset. All of the preset options are working and are included for trial purposes with processed audio samples also available before and after using the DrumMaster. There are 3 bands of EQ ranging in frequency bands of [20Hz – 200Hz], [200Hz – 2,000Hz] and [2,000Hz – 16,000Hz]. Bandwidth at each frequency can be as narrow as 50Hz and as wide as 500Hz of the nominated centre frequency. The third parameter is the gain control, where the selected frequency range can be boosted by up to 6dB or cut but the same amount. Figure 3 shows a close up of how these parameters can be easily controlled using 3 virtual knobs in the graphic user interface. Figure 3. Equalization Module
The next step is the gating module. The user can determine an upper threshold, where the gate lets the sound through, and a lower threshold where the gate closes, rejecting any input signal below this number. These threshold values have been normalised resulting in max amplitude of any given input of 1 so that there is uniformity across a variety of recorded materials. For simplicity and easy understanding, these thresholds have been labelled ‘No Sound’ and ‘Sound On’ on the graphical user interface, as seen in figure 4. To the left of the threshold parameters, the hold, attack and release knobs can be set. The nominated attack time (0 – 1 second) is the time in takes to engage the ‘Sound On’ feature i.e. how long until all sound is passed through un-‐attenuated above the upper threshold. The sound will remain on for a minimum hold time after the Sound On has been engaged. Once the input signal drops below the ‘No Sound’ threshold, the output signal is attenuated to 0 over a period of time determined by the ‘Release’ parameter. Figure 4. Noise Gating Module Figure 5. Mix ratio Module
Finally the Mix parameter determines how much of the normalised only input will be summed with the EQed and gated signal. The value is between 0 and 1, 0 being no signal at all, and 1 being the entirety of the signal strength. The graphic mix knob represents these parameters as a percentage for user-‐friendly manipulation as show in figure 5. 4. Evaluation The enhanced performance and audio qualities of the demo audio files can be heard clearly in the included files. Below are some visual representations of how the signal has been modified, it is evident the DrumMaster mixes of the tracks ‘Wicked’ and ‘Sirens’ have more prominent, clear and isolated sound events with less leakage between transients than the original mixes. Figures 6 – 9 show the mixes in the time domain, and can be listened to in the included files named in their headings.
‘Wicked’ by Traffic Experiment (Drum and Bass mix) Fig 6. Original – ‘Wicked_origmix.wav’ Fig 7. DrumMaster – ‘Wicked_DrumMasterMix.wav’
‘Sirens’ by Leaf (Drum and Bass Mix)
Fig 8. Original -‐ ‘Siren_origmix.wav’ Fig 9. DrumMaster – ‘Siren_DrumMasterMix.wav’
Figure 10 and 11 shows how the frequency response of the snare track in Sirens before and after going through the SnareMaster; a preset that uses parameters designed to brighten the snare sound, reduce high frequency spill from the high hats and to boost mid frequency energy so that the snare pokes through the mix. EQing the drum tracks is important so that the frequency spectrum does not become cluttered, and each individual element can situate within an audible frequency range, adding clarity to a mix. Fig 10. Sirens Snare -‐ before SnareMaster Fig 11. Sirens Snare -‐ after SnareMaster
Kokkinis introduced a new tool for drum leakage suppression at the 135th AES Convention, stating, “From a signal processing perspective the only available tool is the noise gate.” The proposed DSP A3 – Advance Audio Analysis is an advance sound separation algorithm, however it does not address the benefits of a natural sounding sound source. He also goes on to state, “in general sound engineers are not trained to use such tools.” (Elias Kokkinis, 2013) This puts the DrumMaster in the perfect position to fill the need of sound engineers with an effective, natural sounding, clarity enhancing DSP for multi-‐tracked drums, all within a simple package and user-‐friendly parameters. By comparison the Tom samples from the demo track ‘Sirens’ were plotted in the time domain in itheir original form, after the TomMaster and after simple Noise Gating alone. It is clear that the bottom signal would sound heavily isolated, and un-‐natural in a mix, while the top would sound muddy and crowed by surrounding drum sounds. The DrumMaster result in the centre uses the TomMaster preset and presents a good combination of the two in a simple step with the added benefit of enhancing the frequency placement of the Toms in the mix. In summary, figure 12 demonstrates the benefit the DrumMaster has over the current available tools for controlling sound leakage in close miced drums and how in can improve isolation of an unprocessed signal. Figure 12. Sirens Toms Left and Right without DrumMaster, with DrumMaster and with gating only Audio files: TomsL.wav / DMTomsL.wav TomsR.wav / DMTomsR.wav
Bibliography Elias Kokkinis, A. T. (2013). A new DSP toll for drum leakage suppression. Convention e-‐Brief 108. 135th, p. 4. New York: Audio Engineering Society. Robjohns, H. (2003, Feb). Recording Drums. Sound On Sound . Sounds All multi-‐track drum samples are provided for educational use only, not for commercial use. ‘Sirens’ by Traffic Experiment Samples provided by: http://www.cambridge-‐mt.com/ms-‐mtk.htm#TrafficExperiment Accessed June 2015 ‘Wicked’ by Leaf Samples provided by: http://www.cambridge-‐mt.com/ms-‐mtk.htm#Leaf Accessed June 2015 ‘Dad’s Glad’ by Blue Lit Moon Samples provided by: http://www.cambridge-‐mt.com/ms-‐mtk.htm#BlueLitMoon Accessed June 2015 Figures Figure 1 created by Meisha Stevens June 2015 Figures 2, 3, 4 and 5 original design by Meisha Stevens, digital design recreation by Seana Seeto June 2015 All other figures created by Meisha Stevens in MATLAB 2014b June 2015