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| aout_lc_module [2016/07/11 16:20] – created psykhaze | aout_lc_module [2016/08/04 23:46] (current) – [Introduction] psykhaze |
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| ====== AOUT_LC Module ====== | ====== AOUT_LC Module ====== |
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| | {{ :neonking:mbhp_aout_lc.jpg?400 |}} |
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| | =====Introduction===== |
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| | The idea behind the AOUT_LC module was to **realize control outputs with parts that are easy to get in most countries**. After a discussion with Karl Dalen about the difficulity to find DIY friendly DACs with a serial interface. He proposed these **{{http://www.midibox.org/users/kd/KDdac.pdf|circuits}}**, and the discrete solution was selected because of the simplicity. |
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| | In difference to the **[[AOUT_Module|AOUT]]** or **[[AOUT_NG]]**, **this project doesn't require a special (expensive and hard to get) DAC IC**, instead **resistors are formed to a discrete R-2R ladder network**. Building such a circuit means a lot of soldering effort, but as the MBHP is indented as a Do-It-Yourself platform, and not as a commercial product, this argument is very weak. However, **the resistor ladder could be replaced by a DAC with parallel interface** //(see Karl's schematics)//, this would still have the advantage, that the DACs are accessible via a serial chain (-> low number of uC pins are allocated), and that the software driver doesn't need to be adapted for a proprietary interface. |
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| | Regarding the quality of the "discrete DAC" outputs it should be mentioned, that it **directly depends on the chosen resistors**. The tolerance should be **1% or lower (Metalfilm) and stable over the typical temperature range**. The results can be dramatically improved by selecting the best matching resistors: just buy a set of 100 pieces , measure the resistance and take the ones with the best matching values for the ladder circuit. |
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| | Another typical problem of cheap DACs (not only of this circuit, but also for cheap integrated DACs) is, that there is some "zipper noise" each time a new voltage is selected. The intensity of this noise depends on the bits which are toggled when a new voltage is selected - the leftmost bits (most signifigant bits) will produce the highest zipper noise. Even I can see this noise on my scope, I haven't noticed an audible effect with a discrete Moog- and CEM3378 filter, yet. Also CV controlled Korg MS-20 oscillators are working well with it. However, this is an explicit warning for those who expect perfect results! |
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| | ===== Parts List / Schematic ===== |
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| | see [[aout_lc_board_parts_list|AOUT_LC Parts List]] |
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| | * {{http://ucapps.de/mbhp/mbhp_aout_lc.pdf|Schematic}}<sup>UCapps</sup> |
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| | ===== Resolution Adjustment ===== |
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| | In order to increase the flexibility, the MBHP_AOUT_LC PCB provides two different resolution options. The first channel can either be controlled with 12bit or 8bit resolution, the second channel with 4bit or 8bit resolution: |
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| | {{ ::neonking:mbhp_aout_lc_res_options.gif |}} |
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| | The 12/4 bit option is used by MIDIbox SID (the filter is controlled with 12bit, the resonance with 4bit only). |
| | In the MIDIbox CV firmware the resolution option is selectable. 8/8bit is enough if the module should control V/Oct CV inputs via Note or CC events. 12/4bit should be selected if channel #1 is connected to a Hz/V input, or if high-resolution events (NRPNs, PitchWheel) are desired. |
