A multimodal model works with two or more kinds of data, such as text, images, audio, and video. A multimodal large language model (MLLM) connects these inputs so that information from one can inform its interpretation of another. For example, a product-catalog assistant can use both a written request and a reference image to find a relevant item. section 6.0 section 6.1
The main engineering problem is alignment: pixels, text tokens, waveforms, and video frames do not arrive in the same form. Modality-specific encoders turn inputs into numerical features, which are then aligned for joint processing. An MLLM may also use modality-specific decoders to produce nontext output. section 6.2 section 6.1
Two architectural choices differ in how nontext features reach the language model:
- Converter-based alignment maps encoded features into the LLM’s token embedding space. It is comparatively simple and efficient. section 6.2
- Perceiver-based alignment makes multimodal tokens available through cross-attention or query-based mechanisms. It permits richer interaction but adds complexity. section 6.2
Table 6.1 Comparison of LLMs and MLLMs
|
Feature
|
LLMs
|
MLLMs
|
|---|---|---|
|
Data processing
|
Text-only
|
Multiple modalities with separate encoders
|
|
Architecture
|
Single transformer backbone
|
Modality encoders + fusion to shared space
|
|
Training objective
|
Next-token prediction
|
Often adds contrastive cross-modal alignment
|
|
Inference cost
|
Quadratic to sequence length
|
Text cost plus modality encoding/decoding
|
|
Modality encoders
|
Not used
|
Encoders for image, audio, video, etc.
|
|
Input projection
|
Text embeddings used directly
|
Projects modality features into token space
|
|
LLM backbone
|
Processes text sequences
|
Processes fused multimodal inputs
|
|
Output projection
|
Generates text only
|
Maps outputs to nontext modalities
|
|
Modality generators
|
None
|
Generates via modality-specific decoders
|
Compare text-only LLM components with the encoders, fusion, projection, and optional generators used by MLLMs. section 6.1
Prompt an image model
Encode an image file as a base64 data URL, then send it alongside “What’s in this image?” to Llama 4 Scout through the Novita API. The returned description identifies visible objects and broader scene context, illustrating image analysis driven by a text prompt. section 6.3.2
Ask about video or audio
A locally loaded Qwen2.5-Omni model can answer a question about actions in a video or read text from a screen recording. For audio-only inference, extract a 16 kHz mono waveform, prepare it with the model’s processor, and prompt for musical features, transcription, or a summary. section 6.3.4 section 6.3.6
These outputs can feed multimodal retrieval-augmented generation: extracted information from video can be indexed for later questions, while a pipeline can combine PDFs, images, tables, and text for cross-modal search. section 6.3.4 section 6.5










