SREV1 Sampling Guide An Introduction to Impulse-response Sampling with the SREV1 Sampling Reverberator
1 Contents 1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 What is Sound Field Sampling? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . What is Convolution? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . What is an Impulse Response? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . What are Impulse & TSP Signals? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Introduction 2 1 Introduction This document provides a basic explanation of sound-field sampling, and a tutorial by which you can actually sample a sound field and audition it on the SREV1. For more detailed information on the SREV1, IRSampler, or IREdit, please refer to the relevant documentation.
What is Sound Field Sampling? 3 As you can see from the following diagram, sound-field sampling consists of “firing” SREV1 test pulses into an acoustic space, thereby energizing the reverberation in that space, which is then picked up by a number of microphones and returned back to the SREV1 for processing. The acquired data can be saved onto PC Card, edited as necessary using IREdit, and then loaded into the SREV1 to create reverb programs.
What is Convolution? 4 What is Convolution? Convolution is a technique for imposing the characteristics of one signal onto another. What the SREV1 does is to convolve the reverberation characteristics of a previously sampled acoustic space (i.e., its impulse-response) onto another audio source, producing the same overall sound that would have been heard had the audio source actually been heard in that acoustic space.
What are Impulse & TSP Signals? 5 What are Impulse & TSP Signals? The SREV1 can generate two types of test signal. First we’ll take a look at the impulse signal. As explained earlier, an impulse has a very short playback duration and a flat response at all frequencies. You can acquire impulse-response data by using an impulse signal, but you can achieve a better S/N performance by using another type of signal, which we’ll talk about later.
What Equipment is Necessary? 6 What Equipment is Necessary? In order to perform sound-field sampling, you’ll need microphones and microphone preamps to pick up the sounds, speakers and power amps to output the test pulses, and the necessary connecting cables. Of course, you’ll need the SREV1, and depending on the type of I/O being used, you may also require A/D and D/A converters to get signals in and out of the SREV1.
Sampling Tutorial 7 2 Sampling Tutorial Assuming that you’ve set up your speakers and microphones and connected your PC to the SREV1 (see page 17 for hookup examples), you’re now ready to “fire” a few pulses and “grab” some samples. We’re assuming that SREV1 I/O is being taken care of by MY4-AD and MY4-DA cards. If you’re using external A/D and D/A converters, your IRSampler I/O assignments will need to be a little different to those specified later.
Configuring IRSampler 8 Sampler (Generator/Trigger window) Set Averaging to “8” Set the Sampling Time to “4500” msec (i.e.
Checking the Generator Output 9 Checking the Generator Output We’re now ready to check the generator output. Click the Start Generator button on the Sampling Control toolbar and verify that the test signal is being output properly from the speaker system. If it’s not, check the connections and generator output assignment. Starting with the level set low, gradually increase the power amp’s input attenuators so that the pulses can be clearly heard.
Using Your Data on the SREV1 10 Using Your Data on the SREV1 1 Put your data in the right folder The SREV1 can only load impulse-response data stored in the “c:\yamaha\srev\data” folder of the PC Card. So the first thing you need to do is to make a new folder called “data” in “x:\yamaha\srev\” and copy your acquired data into it, something you can do on your PC. (Note that “x” refers to the letter assigned to your PC Card while it’s inserted in your PC.
Data Editing 11 3 Data Editing If you use your samples as they are, you may experience the following: • A roaring sound at the end of the reverberation. • An unwanted delay at the beginning of the reverberation. • Direct sound in the sample is making it unusable. • The reverberation is too loud. These issues are addressed in the following sections, along with an explanation of how to specify the reverb time and start point that the SREV1 uses to process your impulse-response data.
Fixing the Roar 12 Fixing the Roar As the reverberation decays, the extraneous noise captured in the impulse-response data becomes relatively louder and can create a roaring sound. This can be seen clearly in IREdit by setting the vertical axis to dB. This issue can be resolved by fading out the extraneous noise at the end of the sample, so that the reverberation decays naturally. You can do this by using the Fade Out function of IREdit.
Removing the Delay at the Beginning of the Reverberation 13 Removing the Delay at the Beginning of the Reverberation Your impulse-response data may contain a spatial delay at the beginning due to the time it takes the pulse to reach the microphone from the speaker. In some cases this may be a hindrance and can be deleted by using the IREdit Cut function. The following image shows a spatial delay region selected for cutting. The following image shows the data after the spatial delay out has been cut.
Deleting the Direct Sound 14 Deleting the Direct Sound In addition to reverberant sound, the acquired data typically contains direct sound that reaches the microphones directly from the speakers. In order to use the Reverb Balance parameter, or to create a mix of dry and wet signals on your mixer, it’s best to remove the direct sound component, which you can do by using the Cut function of IREdit. The following image shows a direct sound region selected for cutting.
Adjusting the Reverberation Level 15 Adjusting the Reverberation Level If you use your samples as they are, you may find that the reverberation produced is too loud relative to the input signal. This can be resolved by editing the header information with IREdit. In IREdit, choose Header from the Tools menu. The Edit Header window appears, displaying various information about the data, as shown below.
Specifying the Reverb Time & Start Point 16 Specifying the Reverb Time & Start Point Impulse-response data files (i.e., TM4 format files) do not specify a reverb time value, so the SREV1 automatically calculates values for the reverb time parameter that is used by the RC-SREV1. This is based on the length of the sampled data, not the length of the file. You can, however, specify a reverb time (RT) by editing the file header information with IREdit.
Hookup Examples 17 4 Hookup Examples MY4-AD/DA I/O System The following example shows a typical SREV1 setup for sampling a hall. Analog I/O on the SREV1 is provided by MY4-AD and MY4-DA cards. The trigger source is set to internal. The generator output is assigned to SREV1 Slot2-1. From there it’s fed via a power amplifier to the speaker system. Carefully placed microphones pick up the reverberating sound, which is fed back to the analog SREV1 inputs via the mic preamp.
AES/EBU I/O System 18 AES/EBU I/O System The following example is essentially the same as the previous system, but with separate A/D and D/A converters connected to the SREV1’s onboard AES/EBU I/O.
Sampling with a Digital Recorder 19 Sampling with a Digital Recorder Instead of sampling directly to an SREV1, a digital recorder can be used. This may be more convenient than using multiple SREV1s to sample four or more channels. Digital recorders are ideal for recording fluctuating waveforms, such as acoustic reverberation, since they provide the kind of simultaneousness across tracks necessary for accurate averaging. Step 1.
Sampling with a Digital Recorder 20 Step 2. Recording on Location At the location the pulses previously recorded on track 1 are played back, while the responses picked up by the microphones are recorded onto tracks 3, 4, and so on. Hall D/A A/D Power Amp Mic Amp TRK 6 TRK 5 TRK 4 TRK 3 TRK 1 Digital Multitrack Recorder Firing pulses FL FR RL RR Step 3.
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