Using Medium Delay

ADVANCED RECORDING PRIMER

CHAPTER 3 - PART 6, BY ROBERT DENNIS

USING MEDIUM DELAY EFFECTS

Medium time delays are those that are are between 20 ms. and 60 ms.

1. With medium and long delay, the comb filter cancellations due to mixing the delayed and un-delayed signal are generally below the frequency range of most instruments.

2. For midrange instruments without a lot of attack (like guitar and vocal) the medium delay is long enough for the ear to hear the direct and delayed signal as separate signals but short enough so the two signals don't sound out of time with each other. As a result, medium delays are used for doubling effects.

Doubling Effects

Hearing Repeats:

To hear a sound "repeat" it is necessary to hear both the original sound (not delayed) mixed with the delayed sound. The minimum delay that will result in hearing two images varies to a great degree and is primarily dependant on how percussive the original signal is.

Very percussive instruments can have a "double hit" when the delay time is about 5 ms. If you took a snare drum and listened 50/50 to the direct and delayed sound with a 4 ms. setting, you would hear one snare. If you increased the time to 8 ms. you would begin to hear a "flam" (double-hit).

For a rhythm electric guitar, you will begin to hear a double-hit at about 25 ms., and for a vocal at 30 - 35 ms. For instruments that have practically no attack (like a violin) a double image may not be heard at 50 ms.

Reducing Double Hits

The primary reason for doubling is to make the instrument's sound thicker and perhaps wider. When a double (made with a medium delay) is panned identical to the original signal, the instrument will sound thicker. If the double is panned away from the original, the sound begins to have width added to the thickening of the sound.

For many instruments the "thickening" of the sound best occurs around the 40 ms. delay setting, however, many instruments will start to have a double-hit with this delay time. On a vocal track, hard consonants (such as "t", "p" and "s") will often double-sound. On a guitar track, you will hear the pick sound repeated. Reducing high-frequencies with an equalizer on the double will usually reduce these sounds, allowing the longer delay settings to be used for doubling.

Comb Filter Cancellation In Doubles

How long you can make the delay time for doubling instruments depends on the point at which you start to hear a double-hit, as discussed above. How short you can make the delay time for doubling is determined by the cancellation notches of the comb filter thinning-out the instrument's sound. The minimum delay time will often be close to the point where the third cancellation notch is at the lowest possible frequency that the instrument can generate. Figure 1 shows the cancellation frequencies caused by a comb filter for medium delay times. The first three cancellation notices, the ones that are most apparent, are shown in the chart. Figure 2 shows the lowest delay time that can usually be used with the instrument for doubling without noticeable cancellation.

Delay Time	Notch 1	Notch 2	Notch 3
20 ms.	25 Hz	75 Hz	125 Hz
25 ms.	20 Hz	60 Hz	100 Hz
30 ms.	16.7 Hz	50 Hz	83.5 Hz
40 ms.	12.5 Hz	37.5 Hz	62.5 Hz
50 ms.	10 Hz	30 Hz	50 Hz
60 ms.	8.3 hz	25 Hz	41.7 Hz

Figure 1 - Noticeable Comb Filter Canceling Frequencies

Delay Time	Notch 3	Instrument	Lowest Pitch
20 ms.	125 Hz	Low Male Vocal	160 Hz
30 ms.	83.5 Hz	Rhythm Guitar	80 Hz
60 ms.	41.7 Hz	Bass Guitar	40 Hz

Figure 2 - Minimum Delay for Least Noticeable Comb Filter Canceling

If you are using the minimum delay times for doubling and notice that the lowest notes being sounded start to thin-out, you can put in a slight boost with an equalizer at the "lowest pitch" setting given above.

Doppler Effect

The Doppler Effect is the change in frequency that occurs to a delayed signal when the delay time changes as a waveform is formed. Figure 3 shows a waveform sent through a delay line with the delay time is smoothly increasing. The end of the cycle is delayed longer than the beginning of the waveform giving the effect of time being added to the cycle. The frequency of the waveform at the output of the delay time would be lower. If the delay time were to evenly decrease while the cycle was formed, the delayed signal would have a higher frequency

Figure 3 - Doppler Effect

If the delay time decreases as the cycle is being formed, the result would be a shortening of wavelength (a increase in perceived frequency).

The Doppler effect is commonly heard when a car whizzes by you. As the car is approaching it's sound has a higher pitch because the wavelength is being compressed by the car's changing position. As the car passes you and speeds away, you will hear the pitch of the car's sound decrease because the wavelength is being expanded by the car's changing position.

Realistic Doubles With Slight Delay Time Modulation:

We discussed the use of medium delay times to cause a double image. We discussed the use of delay time modulation with short delays (to get a flange effect) in part 5 of this chapter. When a mild amount of delay time modulation is used the second image varies slightly in pitch and time making it much more distinguishable and realistic. The up and down variation of pitch in the voice is called vibrato and is naturally done by most singers. Care must be exercised with the amount of modulation because with too much depth the pitch change will be too great making the doubled vocal too far out of tune.

Sometimes a small amount of cancellation is apparent with medium delays. This effect can be reduced with an output mix of 60% direct and 40% delay.

Figure 4 shows the front panel settings for a realistic vocal double.

Figure 4 - Front Panel Settings For Realistic Double

Pitch Change Programs:

The delay time modulation used for doubling has delayed signal varying up and down around the normal pitch. This is caused by the sine wave control voltage of the LFO. Units can be designed, however, to create a pitch change by using different LFO waveforms in the delay time modulation.

Figure 5 shows a sawtooth waveform that first jumps up to maximum level and gradually decreases to no level to form one cycle. When the LFO has a sawtooth waveform, the delay time modulation causes the delay time to jump to maximum, gradually decrease, jump to maximum, gradually decrease, over and over. The listener "hears" the pitch of the delayed signal as if the delay time was constantly decreasing. The delay output therefore has a pitch-shift to a higher frequency.

Figure 5

A similar waveform, called a Ramp Wave is shown in figure 6. With this wave as an LFO signal controlling delay time modulation, the delay time is always increasing, causing a pitch-shift of the delayed signal to a lower frequency.

Figure 6

The sudden delay time increase of the sawtooth waveform actually causes a small portion of the wave to be removed. The sudden decrease of delay time in the ramp wave causes a small portion of the waveform to be repeated each cycle. These abrupt changes can cause the delayed signal to have "glitches." Glitches cause the signal to have a mechanical, machine-like quality to them. Many delay effects units with pitch change programs modify the Ramp and Sawtooth waveform to round out the sudden changes in delay time and give a more natural signal as shown in figure 4-12.

Figure 7

The delay times used for pitch change are usually short (5 - 10 ms.) and the unit puts out the delayed signal only. Mixing the pitch-shifted delay signal with the direct signal does not, however, cause any type of phase cancellation. A slight pitch change used with a double image can create a fatter sound.

Thick Vocal Doubles With Pitch Change:

When pitch change programs are used to alter an vocal or instrument's pitch, short time delays are used. When a pitch change is used with medium delay times, a double with a different pitch will be generated. A trick used to thicken a vocal performance is to create a double image that has a a "detuning" of about 3 cents (3% of one semi-tone). This very small pitch change adds dimension to the vocals called thickening. Usually delay times around 20 ms. are used.

Synthesizer manufacturers often provide a thickening of instrument sounds using a delay time around 15 ms. This effect is called "chorusing" and the sound starts to take on the nature of a mild flange.

Chorusing:

Delay devices with a Chorusing sound are based around a delay line setting that is half-way between the typical parameter settings used for a dramatic flange and a realistic double, as shown in the following chart.

	Dramatic Flange	Realistic Double	Chorus
Delay Time	10 ms	40 ms	15 - 25 ms
Modulation	100 %	20%	40 - 60%
Feedback	40 - 50%	0	20 - 25%

Figure 8 - Comparing Parameter Values

The Chorus effect will be obtained with multiple (usually 4 - 6) delay lines, each one referred to as a "voice" The effect has a bigness to it but it couldn't really be described as the very natural sound of many voices (a chorus of voices).