Roman Scherbakov (V_Soft) designed this format. This is a format with detailed header for storing huge register dumps of interrupt synchronized music from different AY-3-8910/12 and YM2149 based platforms in small size files.
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Offset Size Type Description 00 dec 2 bytes Word VTX file identifier (ay or ym) Its chip type by default: this is 7961h or 31073 dec (ay) this is 6D79h or 28025 dec (ym) 02 1 Byte bit Playing mode by default: Bits 0-2 determine stereo mode: 0 – MONO, 1 – ABC, 2 – ACB, 3 – BAC, 4 – BCA, 5 – CAB, 6 – CBA (dec) 03 2 Word Loop VBL number (from zero = beginning of melody). 05 4 Dword AY chip frequency for this melody ( for compatibility with non-Spectrum platforms) Standard values: ZX_Spectrum: 1773400 (or 001B0F58h) Atari: 2000000 (or 001E8480h) Amstrad CPC: 1000000 (or 000F4240h) 09 1 Byte Player frequency (VBL per sec). Standard value 50 = 1/50 sec (20ms). 10 2 bytes Word Year of composition creating. 12 4 Dword Size of not packed data. Needed for unpacking. 16 1-255 NTString String with melody name (1-254 chars), must be ended with zero even if no title at all! ?? 1-255 NTString String with author name (see ^) ?? 1-255 NTString Melody source program (see ^) ?? 1-255 NTString Musical editor name (see ^) ?? 1-255 NTString Comment string (see ^) ?? ?? Array Compressed data packet...
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Also can be found old version of VTX-files (of Vortex plug-in older than 2.00 version). Ay_Emul can to play it too.
Until recently, the VTX format was the most optimal and, in fact, the only one suitable for storing quantized music recorded from emulators (when the original is not available, and the AY-file is not created for some reason).
However, recently, music that does not fit into the framework of the format has begun to appear. The weak point turned out to be the "Player frequency" field, in which it is impossible to save a value of more than 255 Hz. For such music, you have to use the YM6 format.
Moreover, it is impossible to save non-standard frequencies of such ZX Spectrum clones as Pentagon (approximately 48.828 Hz) in this field: after rounding, the playback speed will differ slightly from the original.
VTX-files can be converted from various AY and YM based computers emulators' sound dump files (for ZX Spectrum and MSX this is usually PSG-files).
But you can find emulators with ability to dump music in VTX format too. One of them is Unreal Speccy by SMT, however, there is a significant drawback: in the maximum speed mode, VTX is not recorded.
For high-speed recording of PSG or EPSG with subsequent conversion to VTX (or ZXAY), I can recommend the emulator Z80 Stealth by Mr.Kirill.