ni-media

pcm

The pcm library consists of tools to convert pcm data from / to arithmetic types.

pcm::format

The pcm::format consists of 3 properties:

• number (the underlying representation):
  • signed_integer: signed integer types (int8_t, int16_t, …)
  • unsigned_integer: unsigned integer types (uint8_t, uint16_t, …)
  • floating_point: floating point types (float, double)
• bitwidth (the number of bits used to represent one pcm sample)
  • _8bit: 8 bits per sample
  • _16bit: 16 bits per sample
  • _24bit: 24 bits per sample
  • _32bit: 32 bits per sample
  • _64bit: 64 bits per sample
• endian (the byte endianness):
  • big_endian: most significant byte (MSB) comes first in memory
  • little_endian: least significant byte (LSB) comes first in memory
  • native_endian: the natural endianness of the current platform (either big_endian or little_endian)

A format can be defined at runtime or at compiletime (see pcm::iterator to underatand why this is usefull).

A compiletime format can be created by passing format properties as template parameters to pcm::make_format():

auto compiletime_format = pcm::make_format<pcm::signed_integer,pcm::_8bit, pcm::little_endian>(); 

A runtime format can be created by passing format properties as arguments to pcm::make_format():

auto runtime_format = pcm::make_format(pcm::signed_integer, pcm::_8bit, pcm::little_endian); 

pcm::iterator

The pcm::iterator allows to iterate and convert from / to pcm data in a generic, type safe, and performant manner. It wraps a char itererator interfacing the raw pcm data. The pcm::iterator’s value_type can freely be chosen to any arithmetic type, and a client will interact with it as if it was iterating through a sequence of that value_type.

The best way to create a pcm::iterator is by using the helper function pcm::make_iterator():

template <typename Value, typename Iterator, typename Format>
auto make_iterator( Iterator it, Format f );

The three template parameters are:

• Value: the value_type of the pcm::iterator
• Iterator: the wrapped iterator (value_type==char)
• Format: the pcm::format. This can be a runtime or compiletime format.

Depending on the Format, pcm::make_iterator() will return either a runtime or compiletime pcm::iterator.

This will create a compiletime pcm::iterator:

std::vector<char> pcm_data(256);

auto compiletime_format = pcm::make_format<pcm::signed_integer,pcm::_8bit, pcm::little_endian>(); 

auto compiletime_iterator = pcm::make_iterator<float>(pcm_data.begin(), compiletime_format);

This will create a runtime pcm::iterator:

std::vector<char> pcm_data(256);

auto runtime_format = pcm::make_format(pcm::signed_integer,pcm::_8bit, pcm::little_endian); 

auto runtime_iterator = pcm::make_iterator<float>(pcm_data.begin(), runtime_format);

From a client’s perspective both runtime and compiletime iterators behave identically. The only difference is that the runtime version will be less performant than the compiletime version, due to function indirections caused by the required type erasure).

So, why even provide a runtime version if it is less performant than the compiletime version?

Often the pcm::format is not known at compiletime. I.e. when opening an audio file using an audio::ifstream the format is only known once the file has been opened.

In order to still achieve best possible performance, the pcm library provides some algorithms that will be (almost) as performant when used with the runtime iterator as when used with the compiletime iterator.

pcm::algorithms

The following algorithms can and should be used with the pcm::iterator in order to achieve best performance:

• pcm::copy()
• pcm::copy_n()

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