# Posit AI Weblog: Audio classification with torch

## Variations on a theme

Simple audio classification with Keras, Audio classification with Keras: Looking closer at the non-deep learning parts, Simple audio classification with torch: No, this isn’t the primary publish on this weblog that introduces speech classification utilizing deep studying. With two of these posts (the “utilized” ones) it shares the final setup, the kind of deep-learning structure employed, and the dataset used. With the third, it has in widespread the curiosity within the concepts and ideas concerned. Every of those posts has a distinct focus – must you learn this one?

Effectively, after all I can’t say “no” – all of the extra so as a result of, right here, you have got an abbreviated and condensed model of the chapter on this matter within the forthcoming e-book from CRC Press, *Deep Studying and Scientific Computing with R torch*. By the use of comparability with the earlier publish that used

`torch`

, written by the creator and maintainer of `torchaudio`

, Athos Damiani, vital developments have taken place within the `torch`

ecosystem, the tip end result being that the code obtained rather a lot simpler (particularly within the mannequin coaching half). That mentioned, let’s finish the preamble already, and plunge into the subject!## Inspecting the information

We use the *speech instructions* dataset (Warden (2018)) that comes with `torchaudio`

. The dataset holds recordings of thirty completely different one- or two-syllable phrases, uttered by completely different audio system. There are about 65,000 audio recordsdata total. Our process can be to foretell, from the audio solely, which of thirty potential phrases was pronounced.

We begin by inspecting the information.

```
[1] "mattress" "chicken" "cat" "canine" "down" "eight"
[7] "5" "4" "go" "pleased" "home" "left"
[32] " marvin" "9" "no" "off" "on" "one"
[19] "proper" "seven" "sheila" "six" "cease" "three"
[25] "tree" "two" "up" "wow" "sure" "zero"
```

Choosing a pattern at random, we see that the data we’ll want is contained in 4 properties: `waveform`

, `sample_rate`

, `label_index`

, and `label`

.

The primary, `waveform`

, can be our predictor.

```
pattern <- ds[2000]
dim(pattern$waveform)
```

`[1] 1 16000`

Particular person tensor values are centered at zero, and vary between -1 and 1. There are 16,000 of them, reflecting the truth that the recording lasted for one second, and was registered at (or has been transformed to, by the dataset creators) a charge of 16,000 samples per second. The latter info is saved in `pattern$sample_rate`

:

`[1] 16000`

All recordings have been sampled on the similar charge. Their size nearly at all times equals one second; the – very – few sounds which might be minimally longer we will safely truncate.

Lastly, the goal is saved, in integer kind, in `pattern$label_index`

, the corresponding phrase being obtainable from `pattern$label`

:

```
pattern$label
pattern$label_index
```

```
[1] "chicken"
torch_tensor
2
[ CPULongType{} ]
```

How does this audio sign “look?”

```
library(ggplot2)
df <- data.frame(
x = 1:length(pattern$waveform[1]),
y = as.numeric(pattern$waveform[1])
)
ggplot(df, aes(x = x, y = y)) +
geom_line(measurement = 0.3) +
ggtitle(
paste0(
"The spoken phrase "", pattern$label, "": Sound wave"
)
) +
xlab("time") +
ylab("amplitude") +
theme_minimal()
```

What we see is a sequence of amplitudes, reflecting the sound wave produced by somebody saying “chicken.” Put in another way, we’ve got right here a time sequence of “loudness values.” Even for specialists, guessing *which* phrase resulted in these amplitudes is an unattainable process. That is the place area information is available in. The professional could not be capable of make a lot of the sign *on this illustration*; however they might know a strategy to extra meaningfully signify it.

## Two equal representations

Think about that as a substitute of as a sequence of amplitudes over time, the above wave have been represented in a method that had no details about time in any respect. Subsequent, think about we took that illustration and tried to get well the unique sign. For that to be potential, the brand new illustration would one way or the other must include “simply as a lot” info because the wave we began from. That “simply as a lot” is obtained from the *Fourier Remodel*, and it consists of the magnitudes and section shifts of the completely different *frequencies* that make up the sign.

How, then, does the Fourier-transformed model of the “chicken” sound wave look? We get hold of it by calling `torch_fft_fft()`

(the place `fft`

stands for Quick Fourier Remodel):

```
dft <- torch_fft_fft(pattern$waveform)
dim(dft)
```

`[1] 1 16000`

The size of this tensor is similar; nonetheless, its values should not in chronological order. As a substitute, they signify the *Fourier coefficients*, equivalent to the frequencies contained within the sign. The upper their magnitude, the extra they contribute to the sign:

```
magazine <- torch_abs(dft[1, ])
df <- data.frame(
x = 1:(length(pattern$waveform[1]) / 2),
y = as.numeric(magazine[1:8000])
)
ggplot(df, aes(x = x, y = y)) +
geom_line(measurement = 0.3) +
ggtitle(
paste0(
"The spoken phrase "",
pattern$label,
"": Discrete Fourier Remodel"
)
) +
xlab("frequency") +
ylab("magnitude") +
theme_minimal()
```

From this alternate illustration, we might return to the unique sound wave by taking the frequencies current within the sign, weighting them in line with their coefficients, and including them up. However in sound classification, timing info should certainly matter; we don’t actually wish to throw it away.

## Combining representations: The spectrogram

In actual fact, what actually would assist us is a synthesis of each representations; some kind of “have your cake and eat it, too.” What if we might divide the sign into small chunks, and run the Fourier Remodel on every of them? As you’ll have guessed from this lead-up, this certainly is one thing we will do; and the illustration it creates is known as the *spectrogram*.

With a spectrogram, we nonetheless hold some time-domain info – some, since there’s an unavoidable loss in granularity. However, for every of the time segments, we find out about their spectral composition. There’s an essential level to be made, although. The resolutions we get in *time* versus in *frequency*, respectively, are inversely associated. If we cut up up the indicators into many chunks (referred to as “home windows”), the frequency illustration per window won’t be very fine-grained. Conversely, if we wish to get higher decision within the frequency area, we’ve got to decide on longer home windows, thus shedding details about how spectral composition varies over time. What feels like an enormous downside – and in lots of instances, can be – gained’t be one for us, although, as you’ll see very quickly.

First, although, let’s create and examine such a spectrogram for our instance sign. Within the following code snippet, the dimensions of the – overlapping – home windows is chosen in order to permit for affordable granularity in each the time and the frequency area. We’re left with sixty-three home windows, and, for every window, get hold of 2 hundred fifty-seven coefficients:

```
fft_size <- 512
window_size <- 512
energy <- 0.5
spectrogram <- transform_spectrogram(
n_fft = fft_size,
win_length = window_size,
normalized = TRUE,
energy = energy
)
spec <- spectrogram(pattern$waveform)$squeeze()
dim(spec)
```

`[1] 257 63`

We are able to show the spectrogram visually:

```
bins <- 1:dim(spec)[1]
freqs <- bins / (fft_size / 2 + 1) * pattern$sample_rate
log_freqs <- log10(freqs)
frames <- 1:(dim(spec)[2])
seconds <- (frames / dim(spec)[2]) *
(dim(pattern$waveform$squeeze())[1] / pattern$sample_rate)
image(x = as.numeric(seconds),
y = log_freqs,
z = t(as.matrix(spec)),
ylab = 'log frequency [Hz]',
xlab = 'time [s]',
col = hcl.colors(12, palette = "viridis")
)
foremost <- paste0("Spectrogram, window measurement = ", window_size)
sub <- "Magnitude (sq. root)"
mtext(facet = 3, line = 2, at = 0, adj = 0, cex = 1.3, foremost)
mtext(facet = 3, line = 1, at = 0, adj = 0, cex = 1, sub)
```

We all know that we’ve misplaced some decision in each time and frequency. By displaying the sq. root of the coefficients’ magnitudes, although – and thus, enhancing sensitivity – we have been nonetheless in a position to get hold of an affordable end result. (With the `viridis`

coloration scheme, long-wave shades point out higher-valued coefficients; short-wave ones, the other.)

Lastly, let’s get again to the essential query. If this illustration, by necessity, is a compromise – why, then, would we wish to make use of it? That is the place we take the deep-learning perspective. The spectrogram is a two-dimensional illustration: a picture. With photos, we’ve got entry to a wealthy reservoir of methods and architectures: Amongst all areas deep studying has been profitable in, picture recognition nonetheless stands out. Quickly, you’ll see that for this process, fancy architectures should not even wanted; a simple convnet will do an excellent job.

## Coaching a neural community on spectrograms

We begin by making a `torch::dataset()`

that, ranging from the unique `speechcommand_dataset()`

, computes a spectrogram for each pattern.

```
spectrogram_dataset <- dataset(
inherit = speechcommand_dataset,
initialize = perform(...,
pad_to = 16000,
sampling_rate = 16000,
n_fft = 512,
window_size_seconds = 0.03,
window_stride_seconds = 0.01,
energy = 2) {
self$pad_to <- pad_to
self$window_size_samples <- sampling_rate *
window_size_seconds
self$window_stride_samples <- sampling_rate *
window_stride_seconds
self$energy <- energy
self$spectrogram <- transform_spectrogram(
n_fft = n_fft,
win_length = self$window_size_samples,
hop_length = self$window_stride_samples,
normalized = TRUE,
energy = self$energy
)
tremendous$initialize(...)
},
.getitem = perform(i) {
merchandise <- tremendous$.getitem(i)
x <- merchandise$waveform
# make certain all samples have the identical size (57)
# shorter ones can be padded,
# longer ones can be truncated
x <- nnf_pad(x, pad = c(0, self$pad_to - dim(x)[2]))
x <- x %>% self$spectrogram()
if (is.null(self$energy)) {
# on this case, there's an extra dimension, in place 4,
# that we wish to seem in entrance
# (as a second channel)
x <- x$squeeze()$permute(c(3, 1, 2))
}
y <- merchandise$label_index
list(x = x, y = y)
}
)
```

Within the parameter listing to `spectrogram_dataset()`

, word `energy`

, with a default worth of two. That is the worth that, until instructed in any other case, `torch`

’s `transform_spectrogram()`

will assume that `energy`

ought to have. Below these circumstances, the values that make up the spectrogram are the squared magnitudes of the Fourier coefficients. Utilizing `energy`

, you may change the default, and specify, for instance, that’d you’d like absolute values (`energy = 1`

), every other optimistic worth (akin to `0.5`

, the one we used above to show a concrete instance) – or each the true and imaginary elements of the coefficients (`energy = NULL)`

.

Show-wise, after all, the total complicated illustration is inconvenient; the spectrogram plot would want an extra dimension. However we could nicely ponder whether a neural community might revenue from the extra info contained within the “complete” complicated quantity. In spite of everything, when decreasing to magnitudes we lose the section shifts for the person coefficients, which could include usable info. In actual fact, my assessments confirmed that it did; use of the complicated values resulted in enhanced classification accuracy.

Let’s see what we get from `spectrogram_dataset()`

:

```
ds <- spectrogram_dataset(
root = "~/.torch-datasets",
url = "speech_commands_v0.01",
obtain = TRUE,
energy = NULL
)
dim(ds[1]$x)
```

`[1] 2 257 101`

We’ve got 257 coefficients for 101 home windows; and every coefficient is represented by each its actual and imaginary elements.

Subsequent, we cut up up the information, and instantiate the `dataset()`

and `dataloader()`

objects.

```
train_ids <- sample(
1:length(ds),
measurement = 0.6 * length(ds)
)
valid_ids <- sample(
setdiff(
1:length(ds),
train_ids
),
measurement = 0.2 * length(ds)
)
test_ids <- setdiff(
1:length(ds),
union(train_ids, valid_ids)
)
batch_size <- 128
train_ds <- dataset_subset(ds, indices = train_ids)
train_dl <- dataloader(
train_ds,
batch_size = batch_size, shuffle = TRUE
)
valid_ds <- dataset_subset(ds, indices = valid_ids)
valid_dl <- dataloader(
valid_ds,
batch_size = batch_size
)
test_ds <- dataset_subset(ds, indices = test_ids)
test_dl <- dataloader(test_ds, batch_size = 64)
b <- train_dl %>%
dataloader_make_iter() %>%
dataloader_next()
dim(b$x)
```

`[1] 128 2 257 101`

The mannequin is a simple convnet, with dropout and batch normalization. The actual and imaginary elements of the Fourier coefficients are handed to the mannequin’s preliminary `nn_conv2d()`

as two separate *channels*.

```
mannequin <- nn_module(
initialize = perform() {
self$options <- nn_sequential(
nn_conv2d(2, 32, kernel_size = 3),
nn_batch_norm2d(32),
nn_relu(),
nn_max_pool2d(kernel_size = 2),
nn_dropout2d(p = 0.2),
nn_conv2d(32, 64, kernel_size = 3),
nn_batch_norm2d(64),
nn_relu(),
nn_max_pool2d(kernel_size = 2),
nn_dropout2d(p = 0.2),
nn_conv2d(64, 128, kernel_size = 3),
nn_batch_norm2d(128),
nn_relu(),
nn_max_pool2d(kernel_size = 2),
nn_dropout2d(p = 0.2),
nn_conv2d(128, 256, kernel_size = 3),
nn_batch_norm2d(256),
nn_relu(),
nn_max_pool2d(kernel_size = 2),
nn_dropout2d(p = 0.2),
nn_conv2d(256, 512, kernel_size = 3),
nn_batch_norm2d(512),
nn_relu(),
nn_adaptive_avg_pool2d(c(1, 1)),
nn_dropout2d(p = 0.2)
)
self$classifier <- nn_sequential(
nn_linear(512, 512),
nn_batch_norm1d(512),
nn_relu(),
nn_dropout(p = 0.5),
nn_linear(512, 30)
)
},
ahead = perform(x) {
x <- self$options(x)$squeeze()
x <- self$classifier(x)
x
}
)
```

We subsequent decide an acceptable studying charge:

Primarily based on the plot, I made a decision to make use of 0.01 as a maximal studying charge. Coaching went on for forty epochs.

```
fitted <- mannequin %>%
match(train_dl,
epochs = 50, valid_data = valid_dl,
callbacks = list(
luz_callback_early_stopping(endurance = 3),
luz_callback_lr_scheduler(
lr_one_cycle,
max_lr = 1e-2,
epochs = 50,
steps_per_epoch = length(train_dl),
call_on = "on_batch_end"
),
luz_callback_model_checkpoint(path = "models_complex/"),
luz_callback_csv_logger("logs_complex.csv")
),
verbose = TRUE
)
plot(fitted)
```

Let’s test precise accuracies.

```
"epoch","set","loss","acc"
1,"practice",3.09768574611813,0.12396992171405
1,"legitimate",2.52993751740923,0.284378862793572
2,"practice",2.26747255972008,0.333642356819118
2,"legitimate",1.66693911248562,0.540791100123609
3,"practice",1.62294889937818,0.518464153275649
3,"legitimate",1.11740599192825,0.704882571075402
...
...
38,"practice",0.18717994078312,0.943809229501442
38,"legitimate",0.23587799138006,0.936418417799753
39,"practice",0.19338578602993,0.942882159044087
39,"legitimate",0.230597475945365,0.939431396786156
40,"practice",0.190593419024368,0.942727647301195
40,"legitimate",0.243536252455384,0.936186650185414
```

With thirty courses to differentiate between, a remaining validation-set accuracy of ~0.94 seems like a really first rate end result!

We are able to verify this on the take a look at set:

`consider(fitted, test_dl)`

```
loss: 0.2373
acc: 0.9324
```

An attention-grabbing query is which phrases get confused most frequently. (After all, much more attention-grabbing is how error possibilities are associated to options of the spectrograms – however this, we’ve got to go away to the *true* area specialists. A pleasant method of displaying the confusion matrix is to create an alluvial plot. We see the predictions, on the left, “stream into” the goal slots. (Goal-prediction pairs much less frequent than a thousandth of take a look at set cardinality are hidden.)

## Wrapup

That’s it for at this time! Within the upcoming weeks, count on extra posts drawing on content material from the soon-to-appear CRC e-book, *Deep Studying and Scientific Computing with R torch*. Thanks for studying!

Photograph by alex lauzon on Unsplash

*CoRR*abs/1804.03209. http://arxiv.org/abs/1804.03209.