TTS Riva Magpie Flow (ASqFlow) Programming Guide

The focus of this guide is on using AI Inference Manager to integrate a TTS model into an application. The model is known as Riva Magpie Flow, but the plugin is named A-Squared Flow (ASqFlow), based upon the original name of the model as shipped in NVIGI 1.1.1. To avoid issues with applications upgrading from 1.1.1 to a newer version, the plugin name was retained.

IMPORTANT: This feature is considered experimental in this release. It is subject to significant change in later releases, possibly requring app modifications.

IMPORTANT: This guide might contain pseudo code, for the up to date implementation and source code which can be copy pasted please see the SDK’s Basic command line sample Source Code and Docs. The Basic command-line sample includes the option to pass the results of an LLM query to TTS, and then plays it back.

IMPORTANT NOTE: The CUDA backend (nvigi.plugin.tts.asqflow-ggml.cuda.dll) strongly recommends an NVIDIA R580 driver or newer in order to avoid a potential memory leak if CiG (CUDA in Graphics) is used and the application deletes D3D12 command queues mid-application.

IMPORTANT NOTE: Newer releases of the ASqFlow NVIGI plugin are NOT backwards compatible with older versions of the Riva Magpie Flow model. To avoid compatibility issues, please always use the Riva Magpie Flow model that ships with the SDK release that is being integrated.

A general overview of the components within the cpp ASqFlow implementation, including its capabilities, expected inputs, and outputs is shown in the diagram below:

1.1 INPUT FLOW ARCHITECTURE

The ASqFlow TTS system processes inputs through a multi-stage pipeline as illustrated in the diagram above. This section explains how the inputs flow through the system:

Text-to-Phoneme Processing

1. Input Text Processing

   • The system accepts raw text input that undergoes comprehensive normalization, including but not limited to:

     • Conversion to lowercase

     • Number normalization (e.g., “123” → “one hundred twenty-three”)

     • Date normalization (e.g., “12/25/2023” → “December twenty-fifth, twenty twenty-three”)

     • Abbreviation expansion (e.g., “Dr.” → “Doctor”)

     • Removal of extra whitespaces and formatting cleanup

NOTE: Handling Formatted Text from LLM Outputs

When using text generated by Large Language Models (like GPT), the output may contain formatting symbols that get normalized in undesired ways. For example:

To improve your focus during study sessions, try using the Pomodoro Technique:

* Set a timer for 25 minutes of deep work
* Take a 5-minute break afterward  
* After 4 sessions, take a longer break (15–30 minutes)

In this case, the normalizer will convert * to “asterisk” which may not be the intended speech output. It’s recommended to perform pre-processing on LLM outputs to remove or replace unwanted formatting symbols before passing the text to ASqFlow TTS.

2. Text Chunking

   • The normalized text is intelligently separated into manageable chunks

   • Chunk size can be controlled via minChunkSize and maxChunkSize parameters

   • The algorithm avoids splitting sentences when possible to maintain natural speech

3. Grapheme-to-Phoneme Conversion

   • Each text chunk is converted from written text (graphemes) to phonetic representations (phonemes)

   • Uses both dictionary lookup and neural G2P model for unknown words

   • The default phoneme dictionary is located in the model folder and named ipa_dict_phonemized.txt

   • Custom phoneme dictionaries can extend the default mappings

4. Phoneme Encoding

   • Phonemes are encoded into numerical representations suitable for the tts model

   • This encoding serves as input to both the duration predictor and generator models

Audio Timing and Reference Inputs

5. Duration Prediction

   • A simple formula determines timing for each phoneme

   • This ensures natural speech rhythm and pacing

   • The speed parameter can modify the overall speech rate (0.5-1.5 multiplier)

6. Target Audio Inputs (Optional)

   • Prompt transcription target audio: The transcription (text) of the audio that was used to compute the target spectrogram

   • Target audio spectrogram: Pre-computed spectrograms needs to be provided via kTTSDataSlotInputTargetSpectrogramPath

   • These inputs help guide the voice characteristics and prosody of the generated speech

Model Inference Pipeline

7. Generator Model Processing

   • Combines phoneme encodings and duration predictions

   • Generates mel spectrograms representing the audio characteristics

   • Operates in an iterative loop to refine the spectrograms

   • In GGML backend, advanced samplers have been implemented to reduce iterations to 16 (controlled via n_timesteps parameter)

8. Vocoder Processing

   • Converts mel spectrograms into final audio waveforms

   • Outputs high-quality audio at 22050 Hz sample rate

Chunk-by-Chunk Processing

The system processes text in discrete chunks rather than true streaming. This approach allows:

• Audio playback to begin after the first chunk is completely processed

The chunking mechanism processes each text segment independently, generating complete audio for each chunk before moving to the next one.

1.2 INITIALIZE AND SHUTDOWN

Please read the Programming Guide located in the NVIGI Core package to learn more about initializing and shutting down NVIGI SDK.

2.0 OBTAIN TTS INTERFACE(S)

Next, we need to retrieve TTS’s API interface based on ASqFlow. ASqFlow supports multiple backends:

• TRT Backend: Optimized TensorRT implementation

• GGML CUDA Backend: Experimental GGML-based implementation with additional runtime configuration options and language selection support. The GGML backend provides two model variants:

  • FP16 Model: {33E000D6-35A2-46D8-BCB5-E10F8CA137C0} - Higher precision, better quality

  • Q4 Model: {3D52FDC0-5B6D-48E1-B108-84D308818602} - Quantized model, smaller memory footprint

nvigi::ITTS ittsLocal{};
// Here we are requesting interface for the TRT implementation
if(NVIGI_FAILED(res, nvigiGetInterface(nvigi::plugin::tts::asqflow::trt::kId, ittsLocal))
{
    LOG("NVIGI call failed, code %d", res);
}

// Alternative: GGML CUDA backend
if(NVIGI_FAILED(res, nvigiGetInterface(nvigi::plugin::tts::asqflow::ggml::cuda::kId, ittsLocal))
{
    LOG("NVIGI call failed, code %d", res);
}

2.1 LANGUAGE SUPPORT (GGML Backend Only)

The GGML backend provides support for multiple languages, allowing you to generate speech in different languages by setting the appropriate language code at runtime.

Supported Languages

The GGML plugin reads supported languages exclusively from the model configuration file. The exact set of supported languages varies by model, but commonly include:

• “en”: English (default)

• “en-us”: American English

• “en-uk”: British English

• “es”: Spanish

• “de”: German

The specific languages supported by your model are defined in the languages_supported field of the model configuration file (nvigi.model.config.json). This field contains a JSON array of language codes, for example:

{
  "languages_supported": ["en", "en-us", "en-uk", "es", "de"]
}

If the languages_supported field is not present in the model configuration, the system will default to supporting only English (“en”).

Querying Supported Languages

You can programmatically query the list of supported languages from the capabilities and requirements:

nvigi::TTSCapabilitiesAndRequirements* info{};
getCapsAndRequirements(ittsLocal, params, &info);

if (info->supportedLanguages != nullptr && info->n_languages > 0) {
    for (uint32_t i = 0; i < info->n_languages; ++i) {
        std::cout << "Supported language: " << info->supportedLanguages[i] << std::endl;
    }
}

Setting Language at Runtime

To specify the language for text-to-speech synthesis, set the language parameter in your runtime parameters:

nvigi::TTSASqFlowRuntimeParameters runtime{};
runtime.language = "es";  // Generate Spanish speech

NOTE: Language selection is only available with the GGML backend. The TRT backend does not currently support language selection and will use the default English model.

3.0 CREATE TTS INSTANCE(S)

Now that we have our interface we can use it to create our TTS instance. To do this, we need to provide information about the TTS model we want to use, CPU/GPU resources which are available and various other creation parameters.

Here is an example:

//! Here we are creating two instances for different backends/APIs
//!
//! IMPORTANT: This is totally optional and only used to demonstrate runtime switching between different backends

nvigi::InferenceInstance* ttsInstanceLocal;
{
    //! Creating local instance and providing our D3D12 or VK and CUDA information (all optional)
    //!
    //! This allows host to control how instance interacts with DirectX, Vulkan (if at all) or any existing CUDA contexts (if any)
    //!
    //! Note that providing DirectX/Vulkan information is mandatory if at runtime we expect instance to run on a command list.

    nvigi::TTSCreationParameters params{};
    nvigi::TTSASqFlowCreationParameters paramsAsqflow{};    

    nvigi::CommonCreationParameters common{};
    common.numThreads = myNumCPUThreads; // How many CPU threads is instance allowed to use
    common.vramBudgetMB = myVRAMBudget;  // How much VRAM is instance allowed to occupy
    common.utf8PathToModels = myPathToNVIGIModelRepository; // Path to provided NVIGI model repository (using UTF-8 encoding)
    // Model GUID for ASqFlow model - choose based on your requirements:
    // For TRT backend:
    common.modelGUID = "{81320D1D-DF3C-4CFC-B9FA-4D3FF95FC35F}"; // TRT model
    
    // For GGML backend - two options available:
    // common.modelGUID = "{33E000D6-35A2-46D8-BCB5-E10F8CA137C0}"; // GGML FP16 model (higher quality)
    // common.modelGUID = "{3D52FDC0-5B6D-48E1-B108-84D308818602}"; // GGML Q4 model (smaller memory footprint)
    params.warmUpModels = true; // faster inference, disable it if you want faster creation time. Default True.

    // Asqflow tts parameters
    paramsAsqflow.extendedPhonemesDictPath = "Path to a phoneme dictionary, which will extend the default dictionary present in the model's folder."

    // Note - this is pseudo code; the return value of chain() should always be checked
    params.chain(common);
    params.chain(paramsAsqflow);

    //! Optional but highly recommended if using D3D context, if NOT provided performance might not be optimal
    nvigi::D3D12Parameters d3d12Params{};
    d3d12Params.device = myDevice;
    d3d12Params.queue = myQueue;
    params.chain(d3d12Params);

    //! Query capabilities/models list and find the model we are interested in.
    nvigi::TTSCapabilitiesAndRequirements* info{};
    getCapsAndRequirements(ittsLocal, params1, &info);
    REQUIRE(info != nullptr);
    
    //! GGML Backend: Query supported languages (only available with GGML plugin)
    if (info->supportedLanguages != nullptr && info->n_languages > 0) {
        LOG("Supported languages:");
        for (uint32_t i = 0; i < info->n_languages; ++i) {
            LOG("  - %s", info->supportedLanguages[i]);
        }
    }

    if(NVIGI_FAILED(res, ittsLocal.createInstance(params, &ttsInstanceLocal)))
    {
        LOG("NVIGI call failed, code %d", res);
    }
}

TTSCreationParameters and TTSASqFlowCreationParameters

The TTSCreationParameters structure allows you to specify some parameters for creating a TTS instance. Here are the parameters:

• warmUpModels:

  • Type: bool

  • Description: If set to true, the models will be warmed up during creation, leading to faster inference times. If set to false, the creation time will be faster, but the first inference might be slower. The default value is true.

The TTSASqFlowCreationParameters structure allows you to specify some parameters for creating a Asqflow TTS instance. Here are the parameters:

• extendedPhonemesDictPath:

  • Type: const char*

  • Description: Path to a phoneme dictionary, which will extend the default dictionary. This allows you to provide additional phoneme mappings that are not present in the default dictionary.

TTSASqFlowRuntimeParameters

The TTSASqFlowRuntimeParameters structure allows you to control inference behavior at runtime. Here are the parameters:

• speed:

  • Type: float

  • Description: Speech rate multiplier (0.5-1.5, default: 1.0). Lower values make speech slower, higher values make it faster.

• minChunkSize:

  • Type: int

  • Description: Minimum chunk size in characters for streaming output (default: 100). Lower values provide faster time to first audio but may impact efficiency/quality.

• maxChunkSize:

  • Type: int

  • Description: Maximum chunk size in characters for streaming output (default: 200). The algorithm tries to split text into chunks between minChunkSize and maxChunkSize while avoiding splitting sentences.

• seed:

  • Type: int

  • Description: Random seed for generation (default: -725171668). Controls the randomness of the generation process. Use the same seed for reproducible results.

GGML Backend Specific Parameters:

• n_timesteps:

  • Type: int

  • Description: Number of timesteps for TTS inference (12-32, default: 16). Lower values result in faster inference but potentially lower quality. Higher values improve quality but increase inference time.

• sampler:

  • Type: int

  • Description: Sampler type (0-1, default: 1). 0 = EULER sampler, 1 = DPM++ sampler. DPM++ generally provides better quality but may be slightly slower.

• dpmpp_order:

  • Type: int

  • Description: DPM++ solver order (1-3, default: 2). Higher order can provide better quality but may be slower. Only used when sampler is set to DPM++ (1).

• use_flash_attention:

  • Type: bool

  • Description: Enable flash attention for better performance (default: true). Flash attention can significantly improve memory efficiency and speed.

• language:

  • Type: const char*

  • Description: Language code for input text (default: “en”). Works only with GGML plugin currently. The supported languages are read from the model configuration file’s languages_supported field. You can query the exact list of supported languages from the capabilities and requirements.

IMPORTANT: Providing D3D or Vulkan device and queue is highly recommended to ensure optimal performance

NOTE: NVIGI model repository is provided with the pack in nvigi.models.

NOTE: One can only obtain interface for a feature which is available on user system. Interfaces are valid as long as the underlying plugin is loaded and active.

4.0 SETUP CALLBACK TO RECEIVE INFERRED DATA

In order to receive audio data from the TTS model inference a special callback needs to be setup like this:

// Callback when tts Inference starts sending audio data
playAudioWhenReceivingData = true
// Callback when tts Inference starts sending audio data
auto ttsOnComplete = [](const nvigi::InferenceExecutionContext *ctx, nvigi::InferenceExecutionState state,
                        void *userData) -> nvigi::InferenceExecutionState {
    // In case an error happened
    if (state == nvigi::kInferenceExecutionStateInvalid)
    {
        tts_status.store(state);
        return state;
    }

    if (ctx)
    {
        auto outputData = (OutputData *)userData;
        auto slots = ctx->outputs;
        std::vector<int16_t> tempChunkAudio;
        const nvigi::InferenceDataByteArray *outputAudioData{};
        const nvigi::InferenceDataText *outputTextNormalized{};
        slots->findAndValidateSlot(nvigi::kTTSDataSlotOutputAudio, &outputAudioData);
        slots->findAndValidateSlot(nvigi::kTTSDataSlotOutputTextNormalized, &outputTextNormalized);

        CpuData *cpuBuffer = castTo<CpuData>(outputAudioData->bytes);

        for (int i = 0; i < cpuBuffer->sizeInBytes / 2; i++)
        {
            int16_t value = reinterpret_cast<const int16_t *>(cpuBuffer->buffer)[i];
            outputData->outputAudio.push_back(value);
            tempChunkAudio.push_back(value);
        }

        outputData->outputTextNormalized += outputTextNormalized->getUTF8Text();

        // Create threads to start playing audio
        if (playAudioWhenReceivingData)
        {
            std::lock_guard<std::mutex> lock(mtxAddThreads);
            playAudioThreads.push(std::make_unique<std::thread>(
                std::thread(savePlayAudioData<int16_t>, tempChunkAudio, "", 22050, true, false)));
        }
    }

    tts_status.store(state);
    return state;
};

IMPORTANT: Input and output data slots provided within the execution context are only valid during the callback execution. Host application must be ready to handle callbacks until reaching nvigi::InferenceExecutionStateDone or nvigi::InferenceExecutionStateCancel state.

NOTE: To cancel TTS inference make sure to return nvigi::InferenceExecutionStateCancel state in the callback.

5.0 PREPARE THE EXECUTION CONTEXT AND EXECUTE INFERENCE

Before TTS can be evaluated the nvigi::InferenceExecutionContext needs to be defined. Among other things, this includes specifying input slots.

// Define inputs slots
std::string inputPrompt = "Here an example of imput prompt";
nvigi::InferenceDataTextSTLHelper inputPromptData(inputPrompt);

std::string targetPathSpectrogram = "../../../data/nvigi.test/nvigi.tts/ASqFlow/mel_spectrograms_targets/sample_3_neutral_se.bin";
nvigi::InferenceDataTextSTLHelper inputPathTargetSpectrogram(targetPathSpectrogram);

std::vector<nvigi::InferenceDataSlot> inSlots = { {nvigi::kTTSDataSlotInputText, inputPromptData},
                                    {nvigi::kTTSDataSlotInputTargetSpectrogramPath, inputPathTargetSpectrogram } };
InferenceDataSlotArray inputs = { inSlots.size(), inSlots.data() };


// Define Runtime parameters
nvigi::TTSASqFlowRuntimeParameters runtime{};
runtime.speed = 1.0; // You can adjust the desired speed of the output audio if you like. It is recommended to not go lower than 0.7 and higher than 1.3. The value will be clipped between 0.5 and 1.5.

// GGML backend specific parameters (these apply only to GGML backend)
runtime.n_timesteps = 16;    // Number of timesteps for TTS inference (12-32, default: 16). Lower values = faster inference, higher values = better quality.
runtime.minChunkSize = 100;  // Minimum chunk size in characters for streaming output (default: 100). Lower values = faster time to first audio.
runtime.maxChunkSize = 200;  // Maximum chunk size in characters for streaming output (default: 200).

// Advanced generation parameters (optional)
runtime.seed = -725171668;           // Random seed for reproducible results
runtime.sampler = 1;                 // Use DPM++ sampler (0=EULER, 1=DPM++)
runtime.dpmpp_order = 2;             // DPM++ solver order (1-3, higher = better quality)
runtime.use_flash_attention = true;  // Enable flash attention for better performance
runtime.language = "en";             // Language code for input text (GGML backend only)

// Run inference
nvigi::InferenceExecutionContext ctx{};
ctx.instance = ttsInstanceLocal;
ctx.callback = ttsOnComplete;
ctx.callbackUserData = &outputAudio;
ctx.inputs = &inputs;
ctx.runtimeParameters = runtime;
ctx.outputs = nullptr;

//Evaluate
nvigi::Result res;
res = ctx.instance->evaluate(&ctx);

// Wait until the inference is done
while (!(tts_status == nvigi::kInferenceExecutionStateDone || tts_status == nvigi::kInferenceExecutionStateInvalid)
        && res == nvigi::kResultOk)
    continue;


// If an audio is playing, wait for it to finish and destroy the corresponding threads
while (true){
    std::lock_guard<std::mutex> lock(mtxAddThreads);
    std::unique_ptr<std::thread> thread;
    {
    if (playAudioThreads.empty())
        break;
    thread = std::move(playAudioThreads.front());
    playAudioThreads.pop();
    }

    if (thread->joinable()) {
        thread->join();
    }
}
tts_status.store(nvigi::kInferenceExecutionStateDataPending);

IMPORTANT: The execution context and all provided data (input, output slots) must be valid at the time instance->evaluate is called

IMPORTANT: The host app CANNOT assume that the inference callback will be invoked on the thread that calls instance->evaluate. In addition, inference (and thus callback invocations) is NOT guaranteed to be done when instance->evaluate returns.

6.0 DESTROY INSTANCE(S)

Once TTS is no longer needed each instance should be destroyed like this:

//! Finally, we destroy our instance(s)
if(NVIGI_FAILED(res, ittsLocal.destroyInstance(ttsInstanceLocal))) 
{ 
    LOG("NVIGI call failed, code %d", res);
}

7.0 AVAILABLE FEATURES IN ASQFLOW TTS

Async mode

The asynchronous mode (evaluateAsync) allows you to provide input prompts while processing continues in the background. This is particularly useful if you’re expecting long outputs from a GPT model and need TTS to begin processing before the GPT model has finished responding.

Adding Custom Phonemes

To improve pronunciation accuracy for specific words or add support for words not present in the default dictionary, you can create a custom phoneme dictionary that extends the default ipa_dict_phonemized.txt.

Dictionary Format

The phoneme dictionary uses a simple text format where each line contains a word followed by its IPA (International Phonetic Alphabet) phonetic transcription:

WORD  phonetic_transcription_with_IPA_symbols

Example Entries

ABRAM  ˈ eɪ b ɹ æ m
ABRAM'S  ˈ eɪ b ɹ æ m z
ABRAMCZYK  ˈ eɪ b ɹ ɐ m k z ˌ ɪ k
ABRAMO  ˈ eɪ b ɹ ə m ˌ oʊ
ABRAMOVITZ  ˈ eɪ b ɹ ɐ m ˌ u ː v ɪ ts
ABRAMOWICZ  ˈ eɪ b ɹ ɐ m ˌ oʊ v ɪ t ʃ
ABRAMOWITZ  eɪ b ɹ ˈ æ m oʊ v ˌ ɪ ts
ABRAMS  ˈ eɪ b ɹ æ m z
ABRAMS'S  ˈ eɪ b ɹ æ m z ᵻ z
ABRAMSON  ˈ eɪ b ɹ æ m s ə n

Key Format Rules

• Word: Must be in UPPERCASE

• Spacing: Use spaces to separate the word from phonemes and between individual phonemes

• Stress Markers:

  • ˈ indicates primary stress (placed before the stressed syllable)

  • ˌ indicates secondary stress (placed before the stressed syllable)

• IPA Symbols: Use standard International Phonetic Alphabet symbols for accurate pronunciation

Creating a Custom Dictionary

1. Create a new text file with your custom phoneme mappings following the format above

2. Save with UTF-8 encoding to ensure proper handling of IPA symbols

3. Specify the path during instance creation using the extendedPhonemesDictPath parameter:

nvigi::TTSASqFlowCreationParameters paramsAsqflow{};
paramsAsqflow.extendedPhonemesDictPath = "path/to/your/custom_phonemes.txt";

Understanding IPA Symbols

If you’re unfamiliar with IPA symbols, here are some resources and tips to help you create accurate phonetic transcriptions:

Helpful Resources:

• Reference the default dictionary: Look at ipa_dict_phonemized.txt for examples of similar words

• English IPA chart: Wikipedia’s IPA for English provides a comprehensive guide

• Pronunciation tools: Websites like https://ipa-reader.com/ provide audio pronunciations from IPA

Practical Tips:

1. Start with similar words: Find a word in the default dictionary that sounds similar to your target word

2. Break down syllables: Transcribe each syllable separately, then combine them

3. Test iteratively: Create the entry, test the pronunciation, and refine as needed

Example Process: For the word “NVIDIA”:

1. Break it down: “N-VI-DI-A”

2. Find similar sounds: “N” like in “NO”, “VI” like in “VEE”, “DI” like in “DEE”, “A” like in “AH”

3. Result: ɛ n ˈ v ɪ d i ə (with primary stress on “VI”)

Dictionary Precedence

• Custom dictionary entries will override default dictionary entries for the same word

• Words not found in either dictionary will use the neural G2P model for pronunciation prediction

• The system first checks the custom dictionary, then the default dictionary, then falls back to G2P

8.0 KNOWN LIMITATIONS

Currencies

The current text normalization may have certain limitations when handling currencies.

Words that are not present inside the dictionary

When a word is not found in the dictionary, the system uses a graph-to-phoneme (g2p) model to predict its pronunciation. This model is based on a small neural network, sourced from https://github.com/Kyubyong/g2p. However, the predicted pronunciation may not always match your expectations. You can create a custom phoneme dictionary to extend the default one. Provide the path to this custom dictionary during instance creation using the extendedPhonemesDictPath parameter.

Words may have different pronounciations

Some words in the dictionary have multiple pronunciations. The system will always choose the first one, which may not be the desired pronunciation.