Image MIME Reference
AZV
AirZip Accelerator Image
image/vnd.airzip.accelerator.azv
VS
T.38 Fax over IP (FoIP)
image/t38
T.38
A complete technical comparison of AirZip Accelerator Image and T.38 Fax over IP (FoIP) — covering compression, feature support, browser compatibility, magic bytes, and when to choose each format.
Equal browser support
Feature Support
| Feature | AZV | T.38 |
|---|---|---|
| Transparency (Alpha) | ✕ No | ✕ No |
| Animation Support | ✕ No | ✕ No |
| Progressive Loading | Yes | ✕ No |
| HDR Support | ✕ No | ✕ No |
| EXIF Metadata | ✕ No | ✕ No |
| ICC Color Profile | ✕ No | ✕ No |
AirZip Accelerator Image
image/vnd.airzip.accelerator.azv
Extension
.azv
Container
Proprietary AirZip Container
Compression
Lossy (Proprietary) / Lossless (Proprietary)
Algorithm
AirZip Proprietary Compression
Color Depth
8-bit (Indexed), 24-bit (RGB)
Developed by
AirZip, Inc.
Released
1999
Magic Bytes
Unknown (Proprietary)
T.38 Fax over IP (FoIP)
image/t38
Extension
.t38
Container
UDPTL or TCP (Network Transport)
Compression
Lossless (MH, MR, MMR / T.4, T.6)
Algorithm
MH (Modified Huffman)MR (Modified Read)MMR (Modified Modified Read)
Color Depth
1-bit (Monochrome B&W)
Developed by
ITU-T (International Telecommunication Union)
Released
1998
Magic Bytes
None (Network Stream)
Browser Support Comparison
| Browser | AZV | T.38 |
|---|---|---|
Chrome
|
✕ | ✕ |
Firefox
|
✕ | ✕ |
Safari
|
✕ | ✕ |
Edge
|
✕ | ✕ |
IE (Legacy)
|
✕ | ✕ |
AZV
AZV Strengths
- Historically allowed internet users to browse the web up to 5x faster over 56k dial-up modems by drastically reducing image payload sizes
- Officially registered in the IANA vendor tree, preventing MIME type conflicts on early web proxy servers
Limitations
- Completely obsolete in the modern broadband era
- Closed-source and proprietary, meaning no modern image manipulation libraries (like ImageMagick or libvips) can decode it
- Zero support in standard web browsers
T.38
T.38 Strengths
- Designed specifically to overcome the timing and packet-loss issues that plague traditional fax over standard VoIP audio codecs (like G.711)
- Reduces bandwidth requirements by transmitting fax protocol data rather than uncompressed audio tones
- Universally supported by enterprise SIP trunks and Session Border Controllers (SBCs) for FoIP
Limitations
- Not an actual image file format, causing confusion due to its 'image/' MIME classification
- Extremely complex to implement correctly over lossy UDP networks (relies on UDPTL redundancy mechanisms)
- Zero support in web browsers or standard image viewing software
When to choose which format
AZV
Use AZV when…
- Dial-up Web Acceleration
- Bandwidth Optimization (Historical)
- Legacy Proxy Servers
T.38
Use T.38 when…
- Fax over IP (FoIP) Transmissions
- SIP/SDP Media Negotiation
- VoIP Gateways and PBX Systems
- Enterprise Fax Servers