The alternative to bbd delays : the pt2399

In today’s article, we will talk about PT2399, and compare it to other technologies, especially BBD. Then we’re going to focus on the problem of getting clear and long repeats with these chips. And finally how to improve these problems.

how it works

the pt2399, analog or digital ?

The PT2399 is commonly described as a digital, or analog voiced solution. To understand this term, we will explain what makes it different from a BBD (fully analog) or DSP (fully digital) delay.

First, yes, there’s a digital section in this chip. But it is not used in the same way as the usual digital delay ! Generally, the DSPs in digital delays do most of the job. They create the delay, apply digital filtering, add feedback, simulate saturation to emulate certain kinds of delays. Basically, it is in the DSP that the entire sound of the delay has been programmed with some code lines.

In the PT2399, digitization is only used to delay the signal, nothing else ! The small DSP in the PT2399 stores only a sample of the sound in memory for a few milliseconds. Then it plays it back later without any sound processing added. Then, all the sound processing is done with 100% analog circuit. There are filter stages which are quite similar to those used on the BBD. So you are free to make your own analog circuit to achieve the sound you want. For example add a clipping stage in the feedback to have a real analog saturation !

The PT2399 is therefore much closer to a BBD than a digital delay for sound processing. And as we saw in the previous article, the BBD still samples the signal. So in both cases, the delayed signal doesn’t look as “analog” as we could imagine.

inside the pt2399

As we have just seen, the PT2399 integrates a small DSP with memory, and the analog-to-digital and digital-to-analog converters that go with it. But the chip also integrates the clock that will manage the delay time. It also includes several op-amps that can be used freely ! So that’s one less thing to add on the PCB in addition to the chip.

bbd or pt2399 ?

advantages of the pt2399

The first advantage of the PT2399 is its simplicity of use. Since it integrates many of the components we need, we can make a delay with only the chip and a few components ! Contrary to the BBD which needs a lot of external circuits (clocks, a lot of filtering, compander to reduce the noise…).

The clock frequency is also much higher than the one used with a BBD. This results in less problems with aliasing and audible clock noise.

The second advantage of the PT2399 is the cost. Where a BBD costs from 3€ to more than 20€ for the famous MN3005, you can find a PT2399 between 1 and 2€. And since it already includes the clock and op-amps, the cost is even lower compared to a BBD circuit.

disadvantages

The memory in the PT2399 is very limited, so the more you want long times, the less information the memory will be able to store. The sound quality will therefore be deteriorated over the long delays, causing like the BBD some noise problems.

Another problem is that the digital part cannot be accessed directly. You can’t add external memory to it to improve performance with long times.

Another example, the delay time is only controlled with the current on a pin. This is perfect to control the time with just one potentiometer connected to this pin. But it becomes more complicated if you want to precisely control the time with another external circuit, to add a tap tempo for example. Whereas with the BBD, you can generate the clock with any microcontroller.

the problem of bright and long repeats

Whether you use a PT2399 or a BBD, you face the same problem : the noise generated by the chip. As we saw in the article on the BBD, the noise adds unwanted harmonics. And this noise is more important that the delay is long. It is insignificant when you have a delay of a few milliseconds, but becomes problematic when you reach a few hundred milliseconds. There are some techniques to reduce this noise :

output filtering

Adding a low-pass filter at the output of the chip will cut some of the harmonics produced by the noise. But it will also cut off some of the high frequencies in our signal. We always find them in delays, so we have to choose between having clear but noisy repeats, or less noisy but darker ones. Filtering is the most important part of the sound job in a delay. This is what makes the difference between the different delays on the market.

input filtering

Rather than cutting off the treble at the output of the chip, another technique is to boost the treble before the chip, and attenuate it afterwards. So we keep our initial sound at the output of the delay, but boosting the treble in the chip will better cover the noise.

With the BBDs, we saw that we often used Shelving high-pass and then low-pass filters. On the PT2399, another technique is to simply change the ADC integrator capa between pins 9 and 10, which will have more or less the same effect.

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the compander

The compander does not reduce the noise, but allows to always have a high amplitude signal in the chip. It’ll allow it to somehow better “hide” the noise. So you don’t need to filter the noise as much, and you can make the repeats a little brighter.

The compander is almost always present on BBD circuits, but it is much rarer with the PT2399. So we decided to make a prototype of the Utopia with a compander to see if we could improve the noise on long delays. But in the end it doesn’t have a real advantage. The compander has no important effect on a PT2399.

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stacking several chips in series

Filtering and compander can improve performance a little bit. But if we really want a long delay (more than 500ms), we’ll have to use several chips. This way we can use a shorter time on each chip, which will produce each less noise. But the disadvantage is that it adds components, and thus increases the price of the pedal.

conclusion

With these last articles, we were able to look at the different delay technologies and how they work. So we can better understand what makes the characteristic sound of each technology.

To sum up, the BBD is known to have a dirtier sound, due to the delay created by the series of capas/transistors which deteriorates the sound. It is also generally more filtered, therefore darker. The PT2399, on the other hand, allows to create cleaner and brighter repeats thanks to the delay created digitally, while keeping the warm sound of the analog filtering. In the end, there is no one solution that is better than the other, it all depends on the sound you are looking for.